Best value for me is Baldur's Gate 2. I bought it at full price on the first day of release because I was a HUGE fan of the original, and it blew its predecessor out of the water. The game has lasted me for more than a decade, and it's still going strong. I will never tire of this gem.
Worst value is probably Oblivion. Bought as a recent release and at full price, and I'll admit I've sunk quite a few hours into it, but only desperately trying to like it, not actually having fun. I've given up on the damn thing. If I pay full price for something, I'll give it more than a fair chance. But if it still doesn't work, it makes me all the more bitter.
Runner-up for worst would be Fallout 3. It was slightly better, but not much, and I should've seen that coming (also bought on day 1).