I always loved Undertow but hardly used it my first playthrough. I think an Undertow junkie would really fit in Rapture's environment, all underwater and what not. I think Undertow and Bronco would be more unique since there aren't really any Plasmids that have the same effect.

With possession, it would be pretty neat, though it would be an issue of finding host as I'd assume you can only stay in a host for a limited amount of time. I don't know about objects, but if it were living beings I'd say the possession depends on the willpower of the host, the greater the willpower and consciousness of a host, the easier it is for them to overcome the possession. But this ability opens up the possibility of possessing Big Daddies for long periods of time.

Also, think I'm done with Charlotte XD