I'm gonna have to vote for Erika. In almost every run of FireRed/LeafGreen I like to use a Dodrio, and since you can catch Doduo just west of Celadon, I always have one for her. I always follow my capture of Doduo by beating the gym or the hideout(training Doduo along the way) and even with a freshly caught level 22 - 23 Doduo, I have never had even the slightest trouble beating the whole gym(only leaving to heal once or twice, friggin poison). Type advantage aside, I usually get Doduo to about level 26 or 27 and still beat Erika no problem, so I can only imagine that she'd be far too easy to beat with, say, a Charizard or Pidgeot or whatever, that you've had since the beginning of the game.
Kind of unrelated, but I like this story: One time I beat everything(i.e Silph Co., Koga, and Sabrina) but I forgot to beat Erika, which I usually do as soon as I get to Celadon, so I went and fought the gym... my whole team was in the mid-late 40s, not to mention I had a Fearow and a Charizard... Easiest thing ever.