re: Since 2000, What SEC Team Has Overachieved & Underachieved Most?

The Underachiever is easily Tennessee. There is no debate. In the context of the OP, since 2000, UT is clearly the underachievers. UGA would be the answer if the question was since 1980, but it isn't - and I feel like people are unfairly grading UGA's last 20 years more harshly because it was another 20 years prior to 2000 for their last title - but that doesn't matter when it comes to 2000-2020.

Since 2000, only 11 schools have won national titles. Should UGA be in that group - probably. But there are plenty of blue bloods/powers that haven't won it in the last 20 years - Michigan, ND, Tennessee, Penn State, Nebraska.

UGA has 3 SEC titles. UGA has been a national tile contender in multiple seasons. They were a play or two away from winning a natty. I can think of 4 seasons where UGA at season end was a spot or 2 away from the BCS title game/making playoffs - 2002, 2007, 2018, 2019. Adding in the year they made the natty, that's 5 years out of 20 where they were bonafide contenders - it just hasn't happened for them.

Compare the to Tennessee. 0 SEC titles, and I can't think of a single season where they have been a national title contender at year end (correct me if I am wrong?).

It isn't even a comparison. The only logical argument for saying UGA is the bigger underachiever is if you think UT has deteriorated so much as a program that UGA is now on a completely higher plane and thus has more expectations. Which, if that is the case, would only bolster UT being the bigger underachievers since 2000 as prior to 2000, UGA and UT were definitely on the same tier and if anything, UT was on a higher one. The arguments for UGA are being cloudied by unfairly knocking them for "1980" which is entirely outside the scope of 2000-2020.