Because you're asking about the first six generations, I'm assuming learnsets are also for Gen 6. This also neglects STAB moves because it's kind of a given that the Pokemon have those anyways.
Houndoom has a somewhat colourful movepool, with access to Solar Beam, Shadow Ball, and Sludge Bomb.
Sableye has a strangely diverse physical movepool (though without the stats to back it up). Some notable moves are Dig, Brick Break, Rock Tomb, Aerial Ace, Poison Jab, Power-Up Punch, and Zen Headbutt.
Absol is straight up ridiculous. It gets more options on both sides of the spectrum than it could ever use. Ice Beam, Flamethrower, Thunderbolt, and Shadow Ball are decent special options, but it's stronger physically. Megahorn, Play Rough, Stone Edge, Shadow Claw, and Psycho Cut are all interesting options.
Weavile has fewer options than Absol, but still more than it would ever need. Metal Claw, Brick Break, X-Scissor, Poison Jab, and Power-Up Punch are all unique options Weavile brings to the table.
Darkrai is the first Pokemon on this list that has both a ton of special moves and the stats to use them well. With access to Ice Beam, Thunderbolt, Focus Blast, Psychic, Shadow Ball, Sludge Bomb, and Charge Beam, as well as (theoretically) pre-nerfed Dark Void, Darkrai may be the scariest option you have.
Scrafty might not be your best option, but it's the first Pokemon on this list to get a worthwhile Dragon move (Dragon Claw). It also gets the elemental punches, Head Smash, and Zen Headbutt, so it's not terrible.
Zoroark is a Darkrai-lite. It gets similar special options like Shadow Ball, Flamethrower, Extrasensory, and Focus Blast, but it can also use physical moves like U-turn and Low Sweep effectively.
Bisharp is kind of like Absol but with Steel STAB. Other than that, it has a shallower movepool (but Iron Head!).
Hydreigon learns diverse moves like Flamethrower, Earth Power, Flash Cannon, and Focus Blast on the special side. Its physical sets can run weird options like Steel Wing and Acrobatics, but you'd be better off with options like Stone Edge, U-turn, and the elemental fangs.
I'm pretty sure these are the top eight. Other Pokemon (like Honchkrow and Yveltal) have decent movepools, but are generally outclassed in terms of diversity by one of these eight. I think the strongest is Darkrai thanks to its obscene Special Attack and Speed, but you can definitely mess around with any of these. Good luck!
