I usually don't vote for two Pokemon, but I like them both so much, so... Meh.
I think it's pretty obvious that Bulbasaur's one of my favourite Pokemon. I've been waiting for the day since the FRLG era that the Bulbasaur line would see competitive use - and what better to rise to fame than with a Mega Evolution. Everything about it was done right. The ability for MVenu, the designs, and the non-shiny color scheme. It's shiny color scheme and Ivysaur is quite the letdown, though.
Celebi has a wonderful design, and I don't think more needs to be said. As with most other legendaries, it's faring well in UU-OU due to nice base 100 stats all around and access to a variety of boosting moves, support moves and offensive moves like Leaf Storm and Heal Bell, allowing it to take on a multitude of roles and contributing to it's unpredictability.