I'd have to say:
Kanto: Bulbasaur. Venusaur has decent Defense and HP and good Sp.Defense, and since I prefer defense over offense, Vensuaur instantly becomes a favourite. That Sleep Powder + Leech Seed combo is pretty sweet and Sludge Bomb and Earthquake really hurt Fire and Grass types that think they can come in and ruin Venusaur's fun. Vensuaur can also learn fun moves like Swords Dance and Synthesis, actually making it pretty versatile when it comes to movesets. Venusaur's good Speed is also nice for outspeeding stuff with Sleep Powder. =D
Johto: Cyndaquil. Though I like both Meganium and Feraligatr more, Typhlosion is the most useful of the Johto starters imo. Though Typhlosion's movepool is limited, Flamethrower and Thunderpunch combined with Substitute and Petaya can do many wonderful things. <3 That schmexy base Sp.Attack of 109 combined with almost as schmexy Speed makes this thing a good special sweeper even though Typhlosion would like a bigger special movepool.
Hoenn: Mudkip. Do I really need to say why Mudkip is the best? Not only does Swampert take hits like nothing, but it can also dish out alot of damage with Earthquake and Rock Slide, and for certain Pokémon - Ice Beam. Though I'm not fond of Cursepert myself, a Swampert with Curse can destroy whole teams if used right. However, in my opinion, what makes Swampert truly schmexful is its awesome movepool. Swampert can learn many wonderful moves like Mirror Coat, Counter, Roar, Earthquake, Surf/Hydro Pump, Ice Beam, Curse and Rest (even though it's a very common move.), making it a very versatile Pokémon, which is lurve. <3
Even though Swampert is doubtlessly the most commonly used starter Pokémon, I'd say Venusaur is my favourite starter both in terms of appearance and how well it prefers in competitive battles. That doesn't stop Swampert from being an awesome Pokémon, though.
True... they also can learn the ever so needed Surf. But... I like fire. It's only weaknesses are ground, rock and water... right...? AND CHARIZARD CAN LEARN FLY!
Whopee! Charizard can learn a horrible two turn move with a small chance to miss as a nice little bonus. -.- Fly's base power is 75, Wing Attack's and Aerial Ace's 60. It takes 2 turns for Fly to deal damage worth 75 base power while you can deal damage worth 120 base power in the same time with Wing Attack or Aerial Ace. Fly also makes you vulnerable to Thunder, which really sucks. =\ Therefore Aerial Ace/Wing Attack > Fly.