After doing some card churning, I now have a lot of credit cards to choose from for spending. Here's how I optimise them, arranged in order of spend. In the brackets is the cashback earned on transactions.

Of course, if there are any credit card promotions, I will use the card for that instead. For example, Swensen's has a good discount for HSBC cardholders, so I use HSBC for that.

This post refers to the Singapore context and all money values are in Singapore Dollars unless otherwise stated.

Includes affiliate links.

Local card spending

All of the below refers to credit cards unless otherwise stated. I spend an average of about $700 per month, so my strategy is geared toward that level of spending.

UOB One (3.33%)

Minimum $500/month for a quarter

I prioritise this card mostly for two things:

The quarterly cashback of $50 for every quarter you meet the spending criteria.
The monthly interest on the UOB One Account.

Standard Chartered Smart (6%)

I use it for public transport if I hit the minimum spending for UOB One.
5.6 miles per dollar (mpd) if I so choose
The above link has extra cashback if you spend on the card. If you want to simply sign up, I have a different link here.

UOB Lady's Card (~3%)

With its current UNI$ promotion, it is equivalent to about 3% cashback on selected categor(y/ies), with no minimum spend
Cap at $1000/month for bonuses
I will likely use the UNI$ for donations since that gets a better rate with the UNI$ (almost 5%!)

OCBC Titanium Rewards (2.7%)

Only for spend in the retail category.
Maximum cap is very high, unlikely to hit in a single purchase.
Also good for 4 mpd if I so choose

HSBC Revolution (2.5%)

Only spend in the bonus categories. No minimum spend.
Capped at $1000/month for bonuses
Also good for 4 mpd if I so choose

OCBC Frank (8%)

Only if I am sure I can hit the minimum $800 spend for that month.
Cap at $312.50 local spending on reward category if possible. Otherwise, $25 cashback for $800 spend is still a good deal (3.125%)
Permissive categories.

DBS Live Fresh (5%) - NERFED

New conditions which make it not as good:

Min spend $800
Cap at $50 for shopping cashback and $20 for transport
Can earn more total cashback each month but with fewer categories...

Old conditions:

Only if I am sure I can hit the minimum $600 spend for that month.
Cap at $300 for online spending and $300 for contactless spending.
Permissive categories.

Maybank Platinum Visa (3.33%)

Similar quarterly system to UOB One.
Only if you can hit the minimum spending every month ($300 or $1000)
Permissive categories.

UOB Absolute Cashback (1.7%)

For big ticket spending when amex available
For grab pay top ups to buy insurance via grabpay, if amex available (0.3%)

Citibank Cashback+ (1.6%)

For big ticket spending when amex unavailable
No cap on spending

Foreign currency spending

Calculating markup

I calculate it this way:

Compare prices on xe (SGD -> target currency). I tried to use USD as much as possible.
Compare the rates I received for a certain transaction done in target currency.
If only comparing bank transaction fees without actual trasactions, include any markup from Visa/Mastercard directly as well (~0.48% - 0.55%)

Rates may vary depending on the target currency.

Due to this markup, most of these cards earn NEGATIVE cashback, meaning you pay extra to convert your currency. However, it can't be helped, since it'll be the same for debit cards.

I generally also try to spend any currency I have on hand before converting from SGD.

Discover it Cash Back (0.45% up to 4.45%)

1% cashback on USD and 0.45% on other currencies.
Up to 5% cashback on certain categories.
No minimums. Very very few exclusions.
I have some USD on hand, so this makes sense.
Not a lot of places take Discover cards.
However, I have not tested this yet, so I'm not sure how much the conversion rate marks up the cost. I'm assuming it's comparable to Visa/Mastercard.
Discover is not part of Visa or MasterCard, but are partners with JCB and Diners' Club which have their own foreign transaction fees. They may or may not apply when you spend using Discover.

OCBC Frank (-0.125% - 4%)

$312.5/month if at least $500/month local spend.
Must hit minimum of $800 overall.
8% capped at $25.

After accounting for exchange rates, it's still a good 4% or so cashback, which decreases as you spend more, and becomes a 0.125% markup if you spend $800 worth.

UOB Lady's Card (-0.80%-1.20%)

With its current UNI$ promotion, it is equivalent to about 3% cashback on selected categor(y/ies), with no minimum spend
Cap at $1000/month for bonuses
Only worth it if I use the UNI$ for donations (almost 5%)

DBS Live Fresh (0.325% - 1.20%) - NERFED

Only if I am sure I can hit the minimum $800 spend for that month.
Shopping and transport spend only.

Revolut Debit (-0.27% - 1.2%)

Compared rates for SGD -> USD on Revolut (0.7429) and XE (0.7437), but get different numbers vs mid market
However, must top up before use.
Cap at S$1333 for the 1.5% cashback (I have the Metal plan for one year)

Maybank Platinum Visa (-0.33%)

Compared rates for SGD -> USD (0.7281) and XE (0.7547)
If you can maintain the quarterly spending of $300, this could be worth it.
Useful for monthly recurring payments in other currencies.

YouTrip Debit (-0.6% - -0.368%)

Compared rates between YouTrip for SGD -> USD (0.745669) and XE (0.747657)
However, must top up before use.

Wise Debit (-0.537%)

Compared rates for SGD -> USD on Wise (0.745669) and XE (0.747657), more recent comparison including fees is about 0.537% markup
However, must top up before use.

Trust (-0.583%)

Good for forex because it has no foreign transaction fees.
Allows me to spend money without topping up card.
However, no rewards offered.
Still has the base markup by Visa.
Here's my referral code for Trust card - JGNX632Q

Instarem Debit (-0.812%)

1.8% markup but there's 1% cashback.
Sometimes they run promotions for up to 4% cashback in the form of points.
Will be worth it to spend if cashback > 2%.
Must top up before use.

OCBC Titanium Rewards (-1.10%)

For big ticket retail.
Points convert to cashback - 2.7% roughly (source - spend 3600 ocbc$ to earn $10 cash, and earn 10 ocbc$ points per dollar in blocks of $5)
Equivalent to 1.10% markup on foreign transactions (ocbc + visa)
Not shabby, especially if I don't have the cash added to my accounts or it's a big ticket item.

I won't be mentioning any other cards, since the markup is higher and not really worth it. It may be worth it if you value miles a lot more than cash on hand.

Considered but not used

Below are some cards I have or are considering to get, but don't find myself using much or did not choose to sign up for those.

CIMB Visa Signature (10%)

Cannot hit the $800 minimum spend required for the 10% cashback in categories.
Capped at $20 for bonuses instead of $25 unlike the OCBC Frank card, with fussier transaction rules
Use it sometimes for spending promotions targeted at CIMB cardholders

AMEX True Cashback (1.5%)

1.5% cashback is less than the UOB Absolute Cashback Card
I used the sign-up promotion to get a bit more cashback

DBS Woman's World / UOB Visa Signature (2% - 3.2%)

DBS: 2% cashback equivalent redeemed in $50 blocks - 10 dbs points per $5 spending on online spend.
1 DBS point is worth 1 cent (source under featured rewards)
UOB: 2%-3.2% cashback - 10 UNI$ per $5 spending (redeemed as vouchers with grocery stores, petrol etc)
1 UNI$ is worth 1 cent up to 1.67 cents on donations (source)
Too low cashback compared to existing options.
UOB Visa has a high minimum spend.
I may consider if I am focusing on miles.

How does cashback compare to miles?

Some of the cards above earn points that you can then convert to either cashback or miles. Right now I am prioritising cashback, because the value proposition for miles is not very clear to me, and I don't spend enough to accumulate enough miles to redeem flights. I value miles at 1 cent per mile, so that's about the conversion rate I use to mentally convert into "cashback" per se. However, actual cashback rates are lower than that, so you'd need to account for that difference too.

Why 1 cent per mile? I compared the cost of any flight I can buy online versus the same flight I can redeem on Singapore Airlines. So I compare my cheapest cash option versus the number of miles I need to spend to redeem the entire flight, and calculate from there. I take total miles required to claim a flight, divided by the cheapest option for similar route to get my value of miles. This turns out to be about 1 cent per mile for flights shorter than 6 hours. For long-haul flights, the value is higher - going up to about 2 - 3.5 cents per mile. I would need to save up 2 years' worth of miles to claim just 1 long-haul flight.

Maybe the strategy can be to simply collect points first. Then, I can review yearly if I want to claim for flights or cashback, depending on my needs then.

What about annual fees?

I might cancel some cards if the annual fee is too high or cannot be waived. Meanwhile, I try to pick cards with at least a first year fee waiver, or no fees at all.

Closing thoughts

Looking at the current cards landscape, I feel like I have pretty much all of my bases covered. I'm also surprised that I end up using a large majority of the cards I signed up for here and there, which means I won't be cancelling any of my cards unless I cannot waive the annual fee.