The newer the better as each gen has better upgrades to the original engine and aftermarket parts are still in demand (more so than older models). It would depend on how much money you are looking to drop for one.
Taken from a knowledgable guy on another forum 2000-up celica gts
QUOTE
the gt-s is a very nimble and quick (not fast) car. bolt on will yield you slim but its a good start, your best bet is to turbo, your not going to make much more power running it n/a inless you run very high comp pistons (i.e. 14:1 but you must then use race gas) and build it. IMO you could only get about 300 whp on an n/a 2zz, turboing tho...id never guess how much hp these would get from turboing at 5-7 psi. theyre making about 230-250whp on 5-7 psi on stock internals. pretty impressive for a 4 cylendar.