The first that I can remember is my very first character... a Mind/Force Fields controller that I built to look like an ancient looking mobster... because that's what he was. I remember fighting with him in the back alleys of King's Row, and the moment that I got Stealth for the first time, and saw my character disappear Predator-style. At that point, it was just fun to sneak up right near mobs without them seeing me, and then lock them down with holds.
Nowadays, I never touch Stealth, I've got my Personal Force Field, and am way too impatient for stealthy style gameplay, but back then, that was REALLY fun.