One note, just because United doesnt fly there doesnt mean it should be your final or only destination.

Also, agree that Conrad SIN is very nice. Once there, Thailand and Malaysia great beaches are only 60-90 minutes away(flight). Use UA to get there, then think about a quick jump via low cost carrier to final spot.

KL has a great Hilton(one of the best exec lounges in Asia) and great Doubletree(one of the best you will find).

Also, length of time(week, month ect) and what you like to do, as well as time of year.