Kanto - Caterpie/Pikachu - Pikachu is my second favourite Pokémon, I also like Butterfree. (NU <3)
Johto - Hoppip/Wooper/Mareep/Teddiursa - Hoppip and Wooper are some of my favourite Pokémon. Mareep is just a strong electric type that's cute until it evolves. I'd say Hoppip is weak but awesome, who doesn't want a floating ball of fluff which can sleep kill anything in their team? Also Teddiursa is a winrar with all three punches.
Hoenn - Ralts - I love Ralts, Gardevoir is my favourite Pokémon. I like how Gardevoir can learn Thunderbolt and Psychic, has beastly special attack and can calm mind sweep through the entire Hoenn region.
Sinnoh - Shinx/Pichu - I like Shinx because it's awesome and you can EV train it against Bidoof (6 Hp), Starly (252 Speed) and other Shinx (252 Attack) at the start of the game. I also like Pichu because Pikachu is my second favourite Pokémon.
In Hoenn I sometimes don't capture Ralts because it's extremely difficult to find a Modest/Timid female Ralts. All Ralts must be female because I hate delicious traps, Trapvoir is not cool. If I miss a Ralts out then I will usually get a Latias/Metagross to replace Ralts' role in the team.