Lilligant.
When I first played White, I got a Petilil. I thought "hey, might as well use it. I don't even have six pokemon right now", and went on through the game. Later, got myself a sun stone and evolved it. I looked up the moves it could have, and thought "alright, this could be fun." Shortly after getting a few good TM moves, it levels up enough to learn better moves. I found myself clearing Team Plasma fights with Lilligant, even entire gyms with it.
So, once B/W 2 came out, I grabbed White 2 and once again went after a Lilligant. Just as I remember it, I cleared gym after gym, Plasma after Plasma, and had no trouble winning, except once I fought White Kyurem. I was not prepared for that fight, even though I knew what I was walking in to.
Lilligant fainted, and I lost all motive to fight. I kid you not, I put my 3DS in sleep mode for two days. Later, I opened it back up, still in the fight with Kyurem. I had a revive, but I didn't use it until now. My Absol stood ground, and weakened Kyurem enough that it was in red. It KO'ed my Absol, and I had just Lilligant left, at half health. Kyurem does one of its moves, and drops Lilligant right down to 20~ health remaining. It was my victory; Lilligant used Petal Dance, and dealt a hard hit.
Kyurem did not faint. I was in panic mode, since I was about to lose. "Kyurem used Ice Burn! ...but it missed!" When I saw that, I was overjoyed. Lilligant dealt a final blow, and KO'ed White Kyurem.
This little cutesy plant carried me through both games. It was my best pokemon, in terms of reliability. Something so cute can be so useful? I hadn't known. So, I kept using it all the way to the end of White 2. The first pokemon on my Hall of Fame is a level 58 Lilligant.
Lilligant is my favorite pokemon, due to its reliability and the luck I had in using it.
