Taillow, simply because it learns some good moves and I always seem to be able to sweep gyms with Blaziken and Swellow on my Emerald. Second comes the Pidgey line, because it is crazily useful in RBG, GSC and remakes of these because they are available early and are probably the most powerful pokemon you can get early on. Then comes the Starly line because of the speed, power and Staraptor's Close Combat. Finally comes Hoothoot, because it's special, and it's hard to level it up until it gets a special attack.