hi, that a loaded question... smiling... a lot depends on what you can afford and what is it you want to do ...
if it in your budget and you really really want to do alot of things... then photoshop cs5 is it but it expensive (700 US$) although if you have a student id you can save 2 or 3 hundred bucks ...
and if yo want to do some added stuff with ps cs5 get the extended edition with it...