Try Bellsprout. It's the closest one if you just started the game (route 5). Then you can and evolve it into a Weepinbell/Victreebel. They're both good depending where you are in the game.
If you're farther in the game, then Tangrowth and Exeggutor are also good. (Tangela - Route 21, Exeggcute - Route 26)