Hi guys, Mewtwo007 here. Sinnoh was and still is my favorite region, and I'm happy to talk about the starters here. You'll have to excuse me for jumping straight to the final evolution, but i assume that if you're reading this your main interest is competitive battling rather than "Oh, but it was so cute in its first stage!" Anyways, here we go...
Hands down, the best fire-type starter. It gets the stats where they count, and it gets a pretty good movepool to boot. It can viably run both physical and special sets and is an excellent candidate for a mixed set. It can do lead, special, physical, you name it. With the exception of defense, it does everything well.
Torterra is separated from other grass types by its secondary ground typing, which is both a blessing and a curse. It grants it ground STAB to hit fire and steel, two of grass's biggest problems. On the other hand, it give it a 4x ice weakness and it loses the water resistance that is so often called upon. While Torterra was the starter i picked in my playthrough of platinum, it is far from my favorite. It has reasonable bulk, but can't really use it due to all the weaknesses that come with the grass type. My final verdict is that torterra deserves to be what it is: UU.
Empoleon is a very unique pokemon. It is one of the select few that resists the common water-normal combination, and it can run a very viable agility set. It's no slouch on the defensive side as well, with respectable defensive stats. While they aren't amazing, they're plenty to pull it through. Empoleon occupies a niche, and that is probobly the best reason to use it.