Picking a liveaboard in Phuket is like playing the kids party game "Pin the tail on the donkey". I'd say there are really no bad boats, but there is a wide range of prices, comfort, cabins, length of trip, boat size, availability of special services (nitrox for example)....
Beware the word "luxury". This rarely applies to a liveaboard. There are only a couple operating from Phuket that can truly live up to that description. Otherwise, you need to decide on factors such as :
Price : anything from $400 to $1000 for 4 days
Cabins : 4 bed, 3 bed, twin bunk, double bed?
Bathrooms : private, semi private, shared (some boats have just a couple of bathrooms for 20 people)
Size : boats from 17m to well over 30m in length. Big boats are normally more comfortable.
No. of divers : anything from 6 to 26. Also check on how many divemasters are on board.
The thing is, the diving is great, so you will have a great time on just about any boat, but just be sure you know what to expect before you depart. You may think you're getting a double bed cabin and end up in a bunk room with 4 people.
I wish you happy travels,
Phuket Liveaboards