I think he's already the best ever considering his numbers is such a short amount of time. 12 return TD's in two regular seasons, plus one 108 yard missed FG return TD, plus a TD return on the opening kickoff in the superbowl (so 14 total).
The record for most return TD's is 13, set by Brian Mitchell which he did so over 14 years.
Brian Mitchell played 223 games and 1,070 total returns.
Devin Hester 32 regular-season games and 152 total punt and kickoff returns.