Currently ranked #209 on the Pinside's Top 100 list and the most underrated pin IMHO is Avengers Pro.
Now, I was able to play an Avengers LE at California Extreme last year (I think it was Pinball Refinery's HULK game) and loved it! Since then I have not been able to play one until this week when I got a pro version for myself. This game rocks and is very hard. Some of the tightest shots I have seen on a game. It might be why it's rated so low, but this game does have a lot going on.
X-Men LE/Pro also got a bad rap in the beginning, but people are slowly waking up to see past all of the negative press and realize that it's one of Stern's better games ever! Highly underrated IMHO.
Rob Bell
Robsgameroom.com