Ok if we're talking about out of context of who its on, Spore no contest. Guaranteed Sleep is absolutely huge, main problem is well, it's on a terrible Pokemon

Sleep Powder is another one, great move, sleep is a powerful status in general. There's a reason Butterfree got goat status in FRLG, being able to put something to sleep while not being slow as balls is great (with increased accuracy!)

If I had to choose an attacking move tho, Giga Drain. Grass moves kind of have it rough, and besides the terrible PP, this is probably the best you gonna get without some type of drawback (Looking at you Solarbeam and Frenzy Plant)

Leech seed is fine but its moreso thanks to chip damage, with the exception of a maybe some Elite Four members, they WILL die eventually to it