I wasn't at Woodstock in 1994 for many reasons (being an 11 year old and living in the UK being two of those reasons) but it would have been a pretty special experience. I've watched quite a few of the videos of the festival and here are 3 of my favourites:
Nine Inch Nails. I don't know how you feel but I love this performance: the filth, the rawness, the carnage. Trent said he'd thought it was their best and then when he watched it back thought it was terrible but I think it's amazing. It's a strong opening that leads to a great set.
Green Day. The infamous mudfight with the crowd. The best bit about Green Day is that they always sound exactly like Green Day - great show.
Red Hot Chili Peppers. The lightbulb costumes are moronic but the set is pretty decent - there are some great moments (especially Under The Bridge).