I'm not talking about just plain battling. I'm talking about battling under frontier rules (so no items and all Pokemon's levels are the same, etc.) and maybe contests could be considered battling. So if you like normal battling, then you might choose Red or Blue. If you're good at frontier battling, then you could choose Emerald or Platinum. If you like contests the best in the games, then you could choose Ruby. Or if you're one of those who don't like challenges and want an easy battle, then you could choose a Dexholder in conditions that's not his or her style (for example, Yellow).
Me, I definitely choose Red because he's probably the best battler under normal conditions and in the games I often have to utilize strategy if my levels aren't higher than his. I would also like to match one of my Pokemon with Pika, one of those very few Pokemon who are very powerful by themselves without the need to evolve (unlike Ash's Pikachu and much like Red's Pikachu in the games with its Light Ball to power its moves). I also prefer normal battling over frontier battling because for some reason I always have bad luck in frontier battles in the 30th battle or so.