Ditto remains the everlasting champion of the honour of the least known moves.
Ditto can only learn Transform, however it can still use moves of other Pokemon in battle.
Smeargle can only also learn Sketch, however, same as Ditto, can mimic any other move. And also, in Pokemon Colosseum he can learn Tail Whip, DoubleSlap and Iron Tail through purified moves.
Our runner-up two Pokemon are Caterpie and Weedle, only learning two moves each. Caterpie can only learn String Shot and Tackle, whilst Weedle can learn Poison Sting and String Shot.
But our champion is our 28-formed Unown. Unown can only learn the move 'Hidden Power', a Normal-type move. (Thanks Mewderator for that. xD)
Hope I answered your question. :)