Well, unless there is any specific discount that is applicable only to AMEX, it is not advisable to use your AMEX overseas due to the high admin fee. Even the 5x TP won't help much in this case.
I'd say Maybankard 2 cards are better but it depends on what is more important to you and where do you usually spend in Singapore. Whilst there are indeed more promotions for Citibank credit card in Singapore, the promotions usually can only be found at those high-end shopping malls and it always come with a trick to trick you into spending more.

Agree with Miles, for shops offering discount for Citibank cardholder, then Citibank the best compared to other cards But just want to clarify, I mentioned before that in Singapore noted many shop offering discount for Citibank and I cancelled my Citibank last year so did not get any discount.  Also noticed that DBS/POS give a lot of discount in Singapore but that's a Singapore issued card. I only benefited using my UOB once at Singapore at Somerset 313 last year where they gave voucher for spending with UOB card.
My comments for Maybankard 2 Amex Visa Combo vs Citibank Platinum Cash Back for overseas and even locally I would say Maybankard 2 better. Reasons as follows:

1. MBB rate for Visa/MC lower than Citibank rate.
2. Citibank Platinum Cash back only gives you 0.3% cash back vs Maybankard 2 Visa/Mastercard 1X points so can get returns in terms of vouchers equivalent to 0.5%
3. As for Maybankard 2 Amex vs Maybankard 2 Visa/MC, it will work out roughly the same I think because with MBB 2 Amex you get 5X = 2.5% return with voucher = same rate as Amex admin fee for overseas. Same thing for MBB 2 Visa/MC, 1X TP = 0.5% in terms of voucher = MBB admin rate 0.5% (but ronnie recently posted 1% if not mistaken)

If you really go overseas often, get Maybank Platinum MC where you get 2X points or CIMB World Master Card where you get 2X bonus points or EON Bank Visa Signature where you get 2X points plus 1% cash back.

click here to read my 2nd post on Overses Transactions at my blog.

From above post, you can observe that will the converstion rate for MBB Visa and CIMB MC almost the same for my transactions on 26/11/10. And also OCBC TMC and CIMB WMC rate almost the same.

As for AMEX, the rate varies, don't know how to explain. Below link to 1st post at my blog on overseas transaction. See Amex transactions on 12 Aug 2009, all at Universal Studios LA because AMEX official card and get discount for purchases. The conversion rate varies between 1.57% to 2.53%.

click here to my 1st post at my blog on overseas transaction. Note: Alliance rate have increased since then.

I was in Singapore last month for a day trip where I used my MBB Petronas Visa, MBB MC, DA/CIMB WMC and Alliance MC. check my blog end of month to see comparison between these cards and to compare Visa vs MasterCard rate.

For those interested in EON Visa Signature and Citibank Platinum Cash Back, click here to read more about it.

If you are not a big spender and your spending (no special promotion from Citibank) will be made during weekends, you may use Maybankard 2 Amex which gives you 5% cash back on weekends and 5x TPs. The forex conversion rate for Amex is higher but the net cash back plus TPs you receive after minusing the admin fee on forex conversion still can easily beat other cards. Do note that there is a maximum cash back cap of RM50 per month.

Did you ever click the link and read the details?

1% Real Cash Earnings on your purchases*

Whether it's shopping, dining, petrol or anything else, you will earn 1% cash for every Ringgit spent. E.g. RM100 x 1% = RM1. Spend more, earn more.

Earn interest on your Real Cash Earnings
Your Real Cash Earnings balance will also be earning interest (interest rate is based on the prevailing Hong Leong Bank 3-month fixed deposit rate).

Up to RM1,800 extra Real Cash Earnings on your card's anniversary
Extra earnings are based on your total annual retail spending, up to RM1,800 in real cash.
Total annual retail spending  Bonus
Equal or more than RM38,000  RM280
Equal or more than RM68,000  RM880
Equal or more than RM108,000  RM1,800

Spend more, earn more
This is an example of how your Real Cash Earnings can grow in a year.
Monthly retail spending  RM3,500
Monthly Real Cash Earnings  RM35
Total retail spending in one year  RM42,000
Total Real Cash Earnings in one year  RM420
Extra Real Cash Earnings received on your credit card's anniversary  RM280
Total interest earned (no withdrawals made from earnings account)  RM6.03
Total Real Cash Earnings  RM420 + RM280 + RM6.03 = RM706.03

Your Real Cash Earnings never expire
The cash you earn is yours to use as you wish. Withdrawals can be made anytime at any Hong Leong Bank ATM

Higher credit limit
Enjoy more spending power with a higher credit limit of up to 5x your monthly income

Conditional Free-For-Life card membership
As long as you use your Hong Leong Fortune Credit Card twice a month or spend a minimum of RM8,000 a year, you can continue enjoying all the benefits that come with being a cardmember - free of charge for life.

Global acceptance
Your Hong Leong Fortune Credit Card is backed by Visa, the world's most popular electronic payment card brand that is welcomed by millions of merchants world wide.