I am Groot!
One of my favorite feats of Groot was when the Guardians took on a feral Thanos. Groot actually put some nice shots on Thanos. Thanos was fine afterwards, but the fact that Groot managed to get some shots off at all was pretty impressive. Thanos then retaliated IIRC uppercutting Groot. Groot survived the shot and came out not too beat up.
After that feat, I think Groot could take a baseline or just a bit above baseline savage Hulk. He could also take the Thing, Rhino, and maybe the Blob.
If you give him some heavy artillery that he sometimes carries, he can probably have no problem with a lot of the characters mentioned above.