I'm going to vote for UVA as the hardest undergrauate school, for the following reasons:
1. It only accepts in-state Virginians with very strong credentials (i.e. it's hard to get into, even if you're a Virginian. That's an obvious point, however.
2. Fairfax County Public Schools. It's one of the top school systems in the country. As far as I know, Fairfax County produces stronger college-bound students than elsewhere in Virginia (and probably most other states). I've heard that UVA has put a cap on the number of students it will accept from Fairfax County (although I've never seen this in writing).
3. Despite being a state school, it accepts a high number of out of state students. 40% of my graduating class was out of state. In order to get in to UVA from another state, you have to have substantially stronger credentials than even the qualified in-state students.
4. Tuition. Tuition at UVA is very reasonable. It always has been. Hence, there are a heck of a lot of ivy-league caliber students who attend UVA because it is a better bargain. Tuition is far less, and sometimes they get full rides.
So, if you're the average Virginian attending UVA, you've got lots of competition.