When I used to watch That 70s Show during it's original run, my main crush was definitely Eric. (Topher Grace) I thought he was adorable, and loved his long skinny legs. I was in my early 20's during that time, and my tastes have gone through some changes. I recently caught a few old episodes on TV and although Eric is still cute, it is now all about Kelso. (Ashton Kutcher) Hyde and Fez have certainly gotten sexier to me, but Kelso is hilarious and obscenely hot in his tight 70's t-shirts.
Most of the guys had shirtless scenes, and the writers also managed to frequently get Eric, Fez and Kelso in their undies. Given it was the 70's, the cool high school students mostly sported briefs. I also remember that although the show was on the FOX Network, there was a week bit of butt cleavage in a couple of episodes. Tracking them down was a little difficult but unless I missed one, there were just two. Both Kelso and Hyde, (Danny Masterson) gave us a bit of crack in the shows first couple of seasons.
Eric
Love those skinny legs! Eric was often in his briefs, but his most daring scene was the season 1 episode where the guys were going to streak at a Nixon rally. All but Eric bailed, and although he was 'happy faced', he still showed much of his long lean form. Eric also had a fun scene mooning Donna a couple of seasons later, but the show didn't even give us a crack, keeping a big red ball over his butt.
Fez
Fez (Wilmer Valderrama) was often in his undies, and look pretty hot in his black undies as Dr. Frank-N-Furter in season 2's Halloween episode. I especially liked the scene below in which a drunken Fez strips down, heads out into the snow, then collapses, package in the air, on the couch.
Hyde
I was not a fan of Hyde when I first watched. My problem with Hyde is that he is written as 'the cool one', but in reality is actually pretty mean to most of the others. I gave grown to appreciate his appeal (especially in season 1) during the second go around. The caps from the scene below are actually pretty daring for a network show. In the Halloween episode, Hyde
Rear Windows Fez who is watching with binoculars from next door.
Kelso
Kelso gets naked, or close to naked many times on the show, including a brief it of butt cleavage in the shot below. My favorite scene was the one shown in the last two caps. Kelso is taking off everything that Jackie gave him, which means he ends up in his undies for a fairly long and hilarious scene.