Most definitive to least:
1. Reagan in the 80's
2. Bush 43 in the 2000's
3. Nixon in the '70s (if only for Watergate)
4. Clinton in the '90s
I don't think JFK or LBJ were the most definitive people of the 1960s. LBJ's Presidency consisted of the events that would define the decade, but I don't consider him that definitive and JFK's presidency, culturally speaking was more '50s than it was '60s and as his tragic death fades, so to will his level of cultural/political impact. As for the 50's, I agree with Illiniwek that:
if you include the 1950s then I guess Eisenhower, but he was less of a cultural icon than a steady and honorable leader.
Now finally the 2010s. I honestly think it's too soon to say. Personally, I think Obama and Trump will be equally definitive for the decade, especially if the worst of Trump's ideas/policies get implemented.