If we're talking straight numbers, in GENERAL, the Devilsaur is the best pet for BM in PvE b/c of the self-buffs. This is, typically, referring to longer fights. Wolves are still considered very good, of course, because of their Furious Howl ability which buffs the Hunter.
Keep in mind, how you talent the pet will also make a big difference.
Personally, I have a Devilsaur, Spirit Beast and Wolf. Of those three, the wolf is used the least, out of personal preference. Some people I normally play with request the devilsaur frequently, actually. I think they just like watching him.
For PvP, the "best" is usually considered a crab. I haven't played with one. I worked with a hyena for a while but just wasn't loving it. If I had more stable slots, I'd have kept it and leveled it properly, as their Hamstring ability can be nice crowd control. Right now I'm playing with a Warp Stalker. Thought I'd try it out - no real crowd control benefits, but I do like the warp ability for sneaky attacks.