I'm often hesitant to watch old movies. I've found that my tastes have changed dramatically from my adolescent days, and re-visiting nostalgic treasures might ruin my idealized memories of them. Luckily, it turns out many of my favorites from the 80's to early 90's are much better when I see them again, the most recent being Ghost Busters.
Some other untarnished classics I've seen in recent times include Teenage Mutant Ninja Turtles, The Terminator, Twelve Monkeys, Lethal Weapon, and Pretty Woman. They're a refreshing departure from the bland flavors that Hollywood spits out these days.