Fav. Ferrothorn. Grass is my favorite type with steel a close second. I liked it the instant I saw it. The fact that I can run it on literally any team is just the cherry on the cake. This is the Pokemon that got me to make a trick room team, which is a team archetype I've come to adore.
Least fav. Keldeo. It's not even as much the design as the fact that it's somehow a defining part of generation 5 OU despite only having two usable moves. Say what you will about DPPt Garchomp, but it did not single-handedly bring 4 UU and below Pokemon into OU on the sole merit of tanking it.
