I would have said Men's warfare, except warfare isn't an accomplishment, it's a misfortune and an evil. It's hard to say which race would have had the greatest impact, the elves certainly affected the world greatly, men had a huge impact as well, but men had a much more widespread impact and the longer lasting one. Dwarves had little impact on the grand scale of things, although they had minor and major impacts on the more minor events in the world. And hobbits, very little impact on anything, in fact no impact, until Bilbo, at which point their importance became known, somewhat. So really, not a clue. Elven craftmanship is uncomparable to that of any other race, even dwarves, men's warfare was far better than that of any other race, dwarven industry was the greatest, and hobbits, well, I don't really know if their agriculture would stand to being the best.