Appearance-wise, my favourite mega is a toss-up between sceptile, ampharos, megazard x and pidgeot. In terms of the mega I've had most fun with, venusaur probably takes the cake.
As for my least favourite mega, I don't really like blastoise or latios/latias, and the worst offender is undoubtedly garchomp. It gets worse! Its speed decreases and its power increase is nothing a life orb can't do. Plus its chin is... not my favourite design choice in the world.
As for a mega I'd like to see... I find it odd that only one Kalos 'mon got a mega, given that gen 6 is the mega evolution gen. I think more should've gotten them (the starters, maybe). Mega lapras would be kind of cool, too.