First ask yourself some questions:

1. What do I want to do with it (warm up stuff or cook stuff)?

2. What space do I have, how many people should it serve?

3. Do I want to do other things with it? (Grill or cook complete meals)?  

I have no idea of where you live but here in Switzerland microwaves are so cheap that you can more or less buy the cheapest and try it out.

For the best solution go for size (no good if it won't accomodate your dishes), power (nothing less than 800Watts).

If you want to grill or brown your dishes go for one with a grill and if you want a fancy model go for one with digital programs.

In my experience you usually just heat stuff with it so keep it simple and cheap and you can't go wrong!

Graham.