Which frame should be my first?

I'm still new to the game, and now, after playing exclusively with Volt, I find myself in a position where I can build Rhino, Nekros and Vauban.

I've heard the Rhino is good for survivability, but that hasn't really been an issue so far, so that's not a big seller for me.

Any tips or suggestions? I feel like it's taken forever to get to the point where I could build a frame, so having to choose from 3 is mind boggling.

Nekros looks the coolest to me, so that's how I'm leaning right now.