I did a bit of research, and Bulbasaur
seems like the best option. Here are the major battles you'll find one of these starters handy (tell me if I miss anything, I'll add it in):
Gym Leaders
- Viola- Fire Type move ( Fennekin)
- Grant- Grass Type move (Bulbasaur)
- Korrina- Psychic Type move (Fennekin)
- Ramos- Fire Type move (Fennekin)
- Clemont- Magneton- Fire Type move (Fennekin)
- Valerie- Mawile- Fire Type move (Fennekin); others Poison Type move ( Bulbasaur)
- Olympia- Slowking- Grass Type move ( Bulbasaur)
- Wulfric- Fire Type move ( Fennekin)
Elite Four
- Winston- Fire Type move ( Fennekin)
- Siebold- Grass Type move ( Bulbasaur )
- Drasna- Dragalge- Psychic Type move( Fennekin)
- Malva- Pyroar- Fighting Type move ( Surprise! Fennekin can learn Power-up Punch via TM)
Champion
-Diantha- Haluwcha- Psychic Type move (Fennekin); Gourgeist- Fire Type move ( Fennekin); Aurorus & Tyrantrum- Fighting Type move ( Fennekin)/ Grass Type move ( Bulbasaur < Aurorus only>); Gardevoir- Poison Type move (Bulbasaur)
Rivals
-Calem/ Serena- Absol- Fighting Type move ( Fennekin); Greninja- Fighting Type move ( Fennekin)/ Grass Type move (Bulbasaur)
-Tierno- Roserade- Fire Type move ( Fennekin); Crawdaunt- Fighting Type move ( Fennekin)/ Grass Type move (Bulbasaur)
- Shauna- Chesnaught- Fire Type move ( Fennekin); Delcatty- Fighting Type move ( Fennekin)
Trevor- Florges- Poison Type move (Bulbasaur)
I actually haven't played X and Y, so these helped:
Rivals
Kalos League
Also note: The gen 6 games are quite easy, so if you're going to keep Exp. Share on, you can pick any starter to your liking. Everyone's probably going to be overlevelled anyways. So yeah. I hope I helped!
