I need to know which stater Pokemon will be the best in the long run and..

Well all starters have there ups and downs. Chimchar isn't good for the first gym unless you train it to level 14 and make it a fighting fire type. Chimchar is pretty much okay for the next few gyms so by then you should have some stronger Pokemon. Turtwig is good at the start and okay at the second gym. Hes fine up until the steel, ghost. And ice battles. Piplup's good at the first gym, but bad at the second. Hes also decent enough for the next four. The ice battle will be sort of complicated. You'll need to watch yourself with the electric battle though.

I personally think that Turtwig is the best because of his opposite types [grass/ground] in his final form.

Second to that would be Chimchar because of him being like a Sinnoh-/Monkey-form of Blazekin in his final form.

I personally don't like Piplup too much, but with the right moves, you'll never lose with him being Empoleon.

It depends on what moves you teach it and the rest of your team. For example: had:
Infernape - Rock Climb, Mach Punch, Fire Spin, Flame Wheel
Floatzel - Surf, Focus Blast, Crunch, Swift
Roserade - Petal Dance, Shadow Ball, Mega Drain, Toxic
Dialga - Roar of Time, Dragon Claw, Metal Claw, Ancientpower
Azelf - Extrasensory, Flamethrower, Future Sight, Nasty Plot
Luxray - Spark, Crunch, Thunder, Scary Face

Notice that none of my Pokemon had the same type. That way their weaknesses are not the same so they wont get KOd by one pokemon. But you choose what you want. There really isnt a best one. But the one with the highest defences is Empoleon.