Ok so its not that hard question but it all depends on what you  think :
empoleon-has great special defence and special attack but I don't see it haveing good moves for it use its attack is pretty low and the special moves you can teach its are 
ice beam  surf\hydro pump  ice beam so I can say there are a lot more water types you can catch so I don't recommend empoleon 
infernape-with great speed attack and special attack infernape has lots of uses aspecialy if you breed it to have nesty plot its a killing mechine so its moveset is easy to build notice that there arent much good fire types in platinum 
torretera-probebly the best grass type till gen iv it can cover all weaknesses with one move (stone edge) allthough its slow its bulky eanough to take 3-4 fire punches and its the best chise there is moveset is simple and overall its a great pokemon and there arent many good grass types in the plat game 
so overall I think you should take torretera or infernape since there are much better water types in gen 4 you don't need empoleon