1) Starmie - So symmetric and (sort of) abstract and shiny and stuff.
2) Banette - I just like the concept behind Banette, and the desing of its Mega is pretty neat. Also, the battlecry is cool and just the right amount of creepy. The name is also campy which is a plus for me.
3) Forretress - The first time I saw it in game I was like "idk wtf is this but I love it" and it just stuck with me.
4) Excadrill - I like metal types and I like moles. Plus the design is just simply neat.
5) Noctowl - While the design could have been a mit more imaginative, (on the other hand, Decidueye is just so overdone) I just like owls thiiiis much. It also gives me a nostalgic feel every time I see it because it reminds me of the newly introduced day/night mechanic in G/S/C which I though added a lot to the game back then.
Honorary mention: Golduck. I only watched the first season of the show as a kid, but I remember how much I wanted Misty's Psyduck to evolve. The bastard never did though.