As far as beer called "IPA" whose origin is America, I have to whole heartedly agree with Goose Island IPA and also Dogfish Head 60 Minute IPA (both can be found in Redmonds in Ranelaugh). Victory Hop Devil and Smuttynose are pretty nice, too.
If you're opening the field to American Double IPAs (there's going to be more residual sweetness), there's some really great options. Avery Majaharaja jumps to the top of my list but there's a TON of residual sweetness there. Russian River's Pliny-the-Elder is in a class of it's own with >100 IBUs and 3 dry hop additions; it also has the residual sweetness undercontrol. Stone's Ruination IPA is great.
In my "second tier" I'll add 3 Floyd's Dreadnaught, Bear Republic's Racer 5 IPA, Lagunitas Hop Stoopid, Moyland's Hopsicle, Hoppin Frog's (Ohio) Mean Manalishi, & Port Brewing's Hop 15.
As far as English IPAs... I'll stick with what I know and leave this to others. I REALLY didn't like Greene King and I also didn't care for Marston's Old Empire. I haven't found an English beer labeled "IPA" that I've cared for yet, to be honest. When I want a stronger, hoppy beer I seek out hoppy English "Strong ales" (Maximus Strong Ale is DELICOUS), or I look to Brakspear (the Brakspear "English Pale Ale" is delicious; I don't care what you call it!) or Fullers. -I found "Evolution Derbyshire Golden" from Blue Monkey Brewing Company on cask in Reading and really liked it but it seems that they don't bottle... ;(
Adam