I always preferred Hitmonchan. Both in design and usability because of a combination of having a unique ability called iron fist and with those three elemental punches. Hitmonlee is just so plain because he doesn't really stand out because of his really basic appearance and standard fighting type stats. I would choose Hitmonchan any day.
Hitmonlee learns some pretty badass moves, most notably High Jump Kick, and overall tends to be stronger but that come at a price of accuracy. I personally took that risk early on, but I've slowly crossed over to Hitmoncha- Sawk. It's all about Sawk
Until gen 4 though all of those attacks are incredibly weak. Fire, electric, ice and i ton of other types in gen 1-3 are based off of the special attack. Hitmonchan has great attack but his special is atrocious, so having type advantage is useless because all of the attacks are still weaker than a good neutral fighting move.