Technically the best HM slave is Mew. But most of the time, you won't need HMs. Just switch your HM slave in and out of your PC whenever you need one. Most HMs aren't good in battle, anyway (except Surf).
Psyduck's another good one, he can learn all the water HMs plus flash and strength. I use psyduck for Whirlpool, Waterfall, Strength and Flash, and Farfetch'd for Cut and Fly as HM slaves. I give Surf to one of my main pokemon as it's a good move.
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