So it's just one week and you want exotic over developed? It might help if you also told us (a) whether you want to stay in Thai places or major chains where you have status, (b) whether you want isolated or convenient/developed (shopping malls), (c) whether you want to actually swim or just look at the beach, (d) general budget, (e) whether you're adventurous and experienced or want to keep it simple, and (f) whether you want partying or relaxation.

Krabi - it feels 'small' to me, but is relatively cheap and quiet. I didn't see many people swimming, but maybe it depends on which beach you go to.
Phuket - Many beaches (Paton, Kata, Karon, Kamala, Nai Harn, etc.). Some very nice swimmable quiet beaches (e.g., nai harn) as well as a well-developed beach town (Patong) with lots of restaurants, nightlife, and a shopping mall (but the least exotic choice). Relatively expensive for local transportation due to taxi mafia and relatively expensive restaurants due to high percentage of foreigners. Cheap flights, though. Good access to various tours.
Khao Lak - Mostly isolated resorts, as I recall. Quiet. Seems like a decent choice if you want a particular resort and don't mind resort prices. Might be harder to find day trip options.
Koh Chang - It's worthy of being on the list, but the hardest to get to. Relatively undeveloped.
Hua Hin - The beach isn't particularly swimmable or beautiful. But it is relatively easy to get to from Bangkok (~2.5 hour drive) and relatively inexpensive (10 baht sangtaew will take you many places). We like the Hyatt resort a lot.
Pattaya - The selling point here would be the ease of getting there (maybe 1000 baht by taxi in 1.5 hours), arguably the lowest cost of the places you mentioned, and "interesting" night life. Definitely don't go if that kind of nightlife offends you. The downside is that the swimming isn't good (maybe Jomtiem is better for that) and the beaches aren't as beautiful or clean as any of the other places on your list.
Koh Samui - Maybe my personal favorite if you look past the difficulty of getting there. It's relatively expensive to get there by direct plane (~$100 each way from Bangkok on Thai or Bangkok Air) and inconvenient to take a low cost carrier to a nearby town (then ~1.5 hour minivan and ~45 minute ferry). It has a good combination of things to do, variety, beautiful beaches, swimmable beaches, reasonable cost.

I suppose high season is an extra complication. I've always been able to find something reasonable, though, especially if booking in advance. It's mostly hotels that increase in price.