Namor - Not currently in a series (ok, his LS just started). Had that run-in with Super Skrull aka 'Iron Fist' years back, and had his memory erased right after.
Stark - Too much of a copout. Hope not.
Reed Richards - Stretchy powers? Easiest to replicate. Super science and exploration scarily explained by advanced civilization scouting potential targets. Conspiring with Lyja? Eep, what about his kids?
Black Bolt - Most obscure member of the group, most time offscreen to do stuff. Always moving Attilan. Never talks. Very suspicious.
Dr. Strange - Least likely, due to Sorcerer Supreme complications. And yet, Avengers Disassembled, House of M... Nah...
Professor X - Most likely. Always going back into space to hang out with Lilandra. Telepathically verifying what others are saying is true. Anyone remember Maximum Security? He was training a team of Skrulls, for goodness sakes!
Black Panther - Okay, so he turned down membership. But with all the discrepancies due to the relaunch of his series, I really hope Skrulls can be inserted to make sense of it all.
Other likely candidates: Sentry and Jessica Jones. Continuity implants, very recent and prominent.
DH the Lurker
(See! This is why we need Cosmic Avengers!)
It can't be one of the Illuminati, at least not as Bendis seemed to suggest, during the events of the mini-series.
That would mean one of the most important characters in Marvel has been a Skrull for 30 years. It negates to much.
What's more, several of those characters have been replaced by Skrulls at one point or another over the years by Skrulls
, then it just becomes silly.
Prof. X's chair, on the other hand, is the right mass to hide a Skrull, and look how much it's changed over the years!