If we're just talking the past 20 years.. the top 3 (only considering rookie seasons) are hands down Edge, Freeney and Luck.
Edge had one of the best seasons ever for a rookie RB. Coupled with Peyton maturing and finding his rhythm with Marv, Edge made our offense nearly unstoppable. Helped with one of greatest (I think it is still the biggest turn-around in NFL) seasons in sports history from 3-13 to 13-3. Edge had a bit of talent around him on this team.
Freeney had 13 sacks as a rookie and really got our D started on the track to being a fast D which could hit the opposing QB. We did go from 6-10 to 10-6 that year and Freeney's pass rush was a big part of it.
Luck had one of the best seasons ever for a rookie QB. Took our team from 2-14 to 11-5 and almost seemed like he was doing it single-handed at times.
I'd have to rank them Luck, Edge, Freeney... in large part because I think Luck did the most with the least.
The only other guys in the past 20 years who had a season worth talking (positively) about as a rookie was Joseph Addai who had >1,000 rushing yards, Malik Hooker who was having a great year before injury and Ryan Kelly who was very good aside from injury (Costanzo on the other hand kinda stunk it up as a rookie). Even guys like Dallas Clark, Reggie Wayne, Bob Sanders, etc. weren't very good as rookies.
Jeez, I hope Ballard improves our trends and success rates on first round picks... while many of them didn't play great as a rookie guys like Wayne, D. Clark, B. Sanders, Costanzo went on to have solid careers for us... but jeez we had Addai, Marlin Jackson and Donnie Brown who were both pretty much average/below average for us... and then guys like Hughes, Werner, Dorsett, A. Gonzalez who were complete busts. Rob Morris was OK for a year or 2 later in his career but awful at the beginning and Mike Pollak was never more than just average for us either. I am not counting Kelly or Hooker yet because they're too young... but about 50% of our first round draft picks in the past 20 years have been busts, below-average or teetering on average players for us. We have about 20% who have been studs (Edge, Freeney, Wayne, Luck), and then about 30% who were good players for us but not great (with exception of Sanders who got his career/seasons within career cut short by injury).