I've found Undercity to be the best place on my server. Remember, the best place to go will be specific to the server you are on, and as more people try out pet battles even those places will become more challenging. I spent about 2 hours in Undercity last time I played, at the beginning I was about...

Durotar currently has a phasing bug that makes it nearly impossible to find green paws. When you do find them, they disappear as soon as you get to them. Trying Mulgore now... and I've heard Undercity is a good place to go.

So I guess it's still bugged...

I've been trying to figure this out for over an hour. I have track pets enabled, I've trained at the battle master. I have my combat pet selected and summoned. Yet I can find no way to initiate combat. Help, please?