i could go on for hours about how i love every pokemon in some sense but i've always liked Igglybuff. some of my best combo concepts came from it's vast move pool and light aerodynamic capabilities. still can't believe it can't learn rollout... but by my logic, maril should be able to use flamethrower by accessing the oil in its tail so who cares xD go from an opponent's seismic toss into a high-speed aerial rollout and strike with a shadow ball-infused aura. won't know what hit em! xD
(if anyone would like to draw that combo or some others i have please let me know!)
I haven't picked up a pokemon game in a long time...but I have to say, my favorite...would have to be Vaporeon. Though assuming you wanted something different from an Eeveelution, I was always partial to Alakazam.
I was never super into pokemon, though i've played one or two of the games, so I have no idea which ones are most popular. My favorite has always been Psyduck though. Somehow I doubt Psyduck is most popular. xD