Unneeded free upgrade available - which phone to get me the most money on resale?

I'm on an AT&T contract (required and paid for by my employer) but I swear by Nexus/GE phones so I don't need the free upgrade I have available. Has anyone run analysis on resale value of phones that I could tap to figure out which phone to get with the sole intention of turning around and selling it (with the goal of making the most money on the turn-around possible)? I've tried searching the web for such a resource myself but I don't think I'm using the right search terms, so if someone could just point me in the right direction, I'd appreciate it. I have cash available if a not-for-free phone has the best overall return.

Thanks in advance for the help.