For me it's the Marvel Cinematic Universe, with the X-Men Cinematic Universe coming in a close second. In both cases, Marvel/Fox went about building up their cinematic universe from the ground up, and didn't try to expand on it until they had a solid foundation on which to build. And with Marvel's Netflix series (Daredevil, Jessica Jones, and soon The Punisher) all being a part of their expanded cinematic universe, there is a wealth of stories and characters to draw upon.
The DC Cinematic Universe had great potential. If DC had tied their very successful TV series (Arrow, The Flash, and Supergirl) into their cinematic universe, it could have been huge. But they screwed it up with BvS: DoJ. Now whatever DC movies they might have planned are going to flop, and then any hopes of a truly epic JLA movie will go unfulfilled.
In it's day, The Universal Monsters Cinematic Universe was considered pretty awesome. Unfortunately many of the remakes of those classic horror films have utterly failed. So a new UMCU? We'll have to wait and see, but I'm not gonna hold my breath on them being any good.
The Hanna-Barbera Expanded Cinematic Universe? This I think depends more on which shows are being crossed over into which. A Scooby Doo/Johnny Quest crossover makes a certain level of sense. But Scooby and/or Johnny Quest crossing with The Flintstones? That's a little harder to pull off and maintain a quality narrative.
An Archie Expanded Cinematic Universe could be interesting (Archie, Josie & The Pussycats, and Sabrina The Teenage Witch). They'd need ONE GOOD movie for each of those properties before going forth and creating an expanded shared universe though.