I'd take out those two.
At the time of drafting, the only consensus "better" picks, were Staal, Crosby, Malkin, Fleury, and (maybe) Whitney.
Wasn't Kasperi was a top-3 European player in the draft? No one doubted his skill was superb.
I'd put him at #5.
I think Pouliot was a reach at the draft somewhat (he was supposed to go mid 1st round according to most mock drafts), but if not right when drafted, he definitely had a higher peak as a prospect after winning CHL D of the year last year. Whitney was also definitely a better prospect, he was a consensus top-10 pick and was one of the highest ranked NA skaters (he was 3rd IIRC).
Esposito was a weird one. He was a top-5 pick to some scouts and a late 1st rounder to others. He finished as the #8 ranked NA skater, but he was ranked #1 before the 2007 season IIRC (he put up 98 points as a 16 year old rookie, probably because he played with Radulov and his 152 points). If we were talking purely about peak ratings as a prospect, Esposito would win. Esposito also fell off the map faster than any other prospect though.
This is a weird question in the OP though. It's really hard to compare prospects from different times. Esposito, while a good prospect, didn't amount to **** in the NHL. Does knowing he didn't accomplish anything affect his rating as a prospect? Same thing with Letang, does him turning into one of the top-10 D in the NHL affect his prospect status? He was highly regarded after his Q career was over (he had an insane playoff run in 2007), but how highly regarded of a prospect was he? It's really hard to look at stuff like this in no hindsight.