Mega Blastoise doesn't "suck" competitively. Sure, it's not top-tier, I'll give you that, but it's a pretty damn big leap to say its bad. It has its niche. Launcher boosted Dark Pulse guarantees it'll get the Rapid Spin since it can now reliably take out ghosts, and Aura Sphere is good for coverage. Also, it has a pretty massive Sp. Atk stat to abuse now. Really, the only thing holding it back is the lack of reliable Recovery.

Mega Bannette on the other hand, now that's a pretty bad Mega. Also, MAlakazam actually deals less damage than regular Alakazam with a Life Orb. So yeah. Oh, and from what I've seen, MLati@s are actually mediocre at best. Soul Dew is a much better item for both of them.

As for my favorites, I love Mega Venusaur. Such a boss Pokemon. Hell, it is arguably the best tank in OU. With reliable recovery and only two (uncommon) weaknesses attached to that sexy bulk, MSaur will wreck shit up.

I also like MHoundoom, but I feel it would have been better as an evolution to Houndoom instead of a Mega Evolution.

And I think MGarchomp is what regular Garchomp should've looked like (at least aesthetically.)

Other Megas I like aesthetically are MAbomasnow, MAmpharos, MGardevoir, and MTyranitar.

MManectric has a disappointing design. MSizor and MHeracross are just...ew. Way to ruin two of the most awesome Pokemon in the game.

All the others, I'm 'meh' about. Zard Y looks so much better than Zard X. Really, did we even need Zard X?