I would actually go as far to say A-rank for torterra!
Basically the reason for this is because he can run loads of sets effectively, making it extremely versatile, and most of the sets he can use are great on lots of the tier.
For instance, he can run a great band set; wood hammer, EQ, stone edge and then something like iron head or outrage depending on your weaknesses. Aside from that, he can run a great bulky stealth rockers set; synthesis, rocks, wood hammer/EQ/Stone edge/Toxic, Wood hammer/EQ/stone edge/Protect (if you used toxic in other slot). He can even toxic stall on some physical walls with toxic, protect, synthesis, wood hammer/EQ.
So as you can see, torterra is an extremely versatile pokemon with every handy set having various options. The opponent will never know what to expect, and the stats it has makes it a worthy member on your team if you are stuck for options, and that is why I think Torterra for A!