I think bulba is the best one: op tank with healing, stall, types that cover one another. This combo: sleep powder + leech seed + giga drain + synthetize + leftover with spdef and hp trained makes him unfair, easily kill even types that was suppose to counter him. Leveling isn't though as you said, since it'll beat the 4 first gym without any effort: +4 offense bonus on 1st gym, +2 ofense and defense on 2nd one, defense bonus on 3rd and defense bonus on 4th. It'll naturaly level in those gyms until you reach water spots and that opens leveling possibilites for another pokes of ur team on field travels. Of course there are many bird, poison and bug types that may check bulba, but a team aren't made by just one poke, a simple rock type will cover all of that and you can catch really early on the game, so no problem at all.