When you mean is Torterra the best to get in platinum do you mean starters? If so than the choice is purely up to you. I'll just provide information on the 3 final evolution of the starters and my opinion.
Total base stat:
Well even though Torterra has the lowest total base stat at 525 it can boast having the highest Hp, Att, and Def of the three. The only downfall of Torterra is it's poor speed that can really hurt you in close battles that require just 1 more attack from either sides.
Number of Weaknesses and Resistance:
Torterra:4 Weaknesses and 3 Resistance
Empoleon: 3 Weaknesses and 12 Resistance
Infernape: 4 Weaknesses and 6 Resistance
Here is Torterra's Achilles' heel with only having 3 types of resistance and 4 weaknesses. Compared to the others Torterra is far behind.
Torterra is a weird one. It's a bulky Grass/Ground type with high Att and Def at the cost of very low SpAtt and Spe. It has STAB Earthquake which is a plus and is good at tanking. Overall Torterra is a pretty decent pokemon that can hold it's own against anyone.(except ice pokemon! 4'x weakness!!)
and to answer your other question about how to beat garchomp. Just get a pokemon with ice beam/blizzard or anything ice because garchomp has a 4'x weakness to ice.