Re: OP. Have been to all the places on the list. Do note that May is the hottest time of the year for most of them, though that does keep the crowds and prices down.

Phnom Penh has the grand palace and the Pol Pot-era sites, and I'm sure there are a lot more food choices now and it's a lot tamer than the place I remember that resembled what was described in the book "Off the Rails in Phnom Penh" (a good one to pick up if you can find). A drink at the FCCC while watching boats on the river is a must-try experience.

IMHO Phuket is past the point of no return on crowds and getting ruined. It's so bad that the roads often have traffic jams and back-ups, and they've run out of room to build in Patong. I was lucky to get to see it, Ko Phi Phi and Ko Samui all in 1995, and by comparison today they're all overbuilt. The chase for the unspoiled beach has moved on, and arguably even Ko Samet, Ko Lanta and Ko Chang are too built up for that now, though there are other places in both the east and Trang province that are only more recently touristed. It'll be hard to beat the Perhentians, though. For the southern Thailand experience I'm partial toward Krabi, though again it's gotten much more crowded in the past decade.

It's pretty easy to fill 4 days in Bangkok. KL is not as rich in possibilities but there's also enough for that timeframe if you research it, and KL is also rich on the shopping.

I liked Kuching and it has a nice atmosphere, and Bako National Park makes a good sidetrip. Didn't get upriver to the longhouses but did make it out to the heritage center.

KK, OTOH, was flattened in WWII and seemed pretty drab and boring. You could use it as a base for TAR National park (islands and all that) or maybe head to Mt. Kinabalu. I decamped to Sandakan instead (MAS fares were $25 each way and I made sure to sit in a window seat on the mountain view side of the plane). With Sandakan you have that orang-utan rehabilitation center and tours to the lower Kinabatangan. I took a 2-day, 1-night one from Uncle Tan's for $56 that was unforgettable, with a beautiful trip in and trip out and a night boat ride that spotted wildlife like snakes hanging from trees, and even a couple of orang-utans in the wild (which are supposed to be rare but had gotten more common). The real ecological villains are the palm oil plantations in the higher elevations.

I don't often hear Singapore described as a favorite city in the world, even though it undeniably runs well. I'll usually go see the thieves market near Jln. Besar, and I did go to that FT DO that one year where everyone ate black pepper crab and chili crab.