I would rate the capital cities as follows :-

1) SIN (if cost of accommodation is an issue, BKK takes the top spot)
2) BKK
3) KUL
4) CGK
5) HAN/SGN


SIN & BKK are major hubs, practically every airline operate from these airports. Few airlines fly to KUL which is dominated by the Air Asia-MH 'cartel' controlled by TF, meaning very few choices, higher fares and putting up with TF nonsense.

Hanoi may be a nice city to visit (and I usually do that several times each year), but it lacks the proper insfrastructure, convenience and facilities expected of a major city (which is the reason I rate it even below Jakarta). Vietnamese food is a little bland (not to say that I don't enjoy it) and after a week in Vietnam you would be yearning for something more "stimulating'. It's also not very pleasant dealing with the local business people, they are just too money crazy and look at foreigners as opportunities to make big money.

While I like Chiang Mai, as well as other secondary cities like Penang and Bandung; it's not very convenient if you need to fly around the region frequently. Nice for retirement or to raise a family, but not for a frequent traveller.

Singapore seems to be the perfect place, everything works well (almost) but cost of accommodation may be a big hindrance for many. Taking everything into consideration, I would probably vote for Bangkok; a major aviation hub, relatively low costs, good infrastructure and facilities, foreigner friendly, good food, very cosmopolitan and it feels exotic at the same time.