Looking at this thread, it seems like alot of people missed the point.
If you got down to it, you could probably make the case for any player to be an underachiever.
Flintoff, KP, VVS, AB, Warner etc. are still players who had substantial careers and if you asked fans about them today, they would still remember them.
I think people should list the names of players they thought possessed alot of talent, but for some reason, their careers did not pan out the way they should have.
The names that come to my mind here are guys like:
Jesse Ryder, Vinod Kambli, Mohammed Asif, Mark Ramprakash, Shaun Tait, Umar Akmal, Asim Kamal...
Someone who scored 7K+ test runs is not really an underachiever no matter how you choose to look at it.