Their Looks
Let's get a bit superficial and talk looks, of both characters and actors.
Let me get even more superficial and start with the women, because they're the real eye candy.
So, as far as I'm concerned, Jen and Courtney were the top hotties (and still are). I'd say Monica was the best-looking one in season 1, then Rachel in 2 to 6, then Monica in 7 to 9, then Rachel again in 10, and right now I'd say it's Courtney again. She's clearly had some work done, but it looks good.
Lisa never did it for me. Only time I thought she looked good was when she worked as a stockbroker.
As to the boys, I'd say they've all more-or-less lost their looks by now. David was never really good-looking to begin with, though he may have been considered attractive in a nonconventional way. LeBlanc lost them gradually throughout the show, and now he's just some overweight dad. For Perry, it was on and off during the show, before going downhill afterwards.