Hi!
That depends a bit on what you're asking for.
Best for mounting as in which one is the horniest and therefore most likely to mount easily?
Or is it what breed is most suitably, physically, for mounting?
Labrador, and from personal experience spaniel's , are among the horniest. Aside from that, it's pretty easy to get a sense of horny breeds from porn, especially amateur.
Good, or great, breeds, psysically, most importantly comes down to yourself. You want a breed that is an appropriate size heightwise based on how high your ass is when you're on all fours.
I'd happily give better answers, or answer more questions, if needed!