I just picked up a Nexus 5 last night. I didn't mind signing up for a new contract and the upgrade cost me $35 total.
My very close second choice was the HTC One. Beautifully designed. It would've been free.
W/no contract, I'd go with the Nexus 5 again due to lower cost.
Also, be sure to check Amazon. Their offers are generally lower than if you went straight to the TMobile, Sprint, etc., site themselves. In your no-contract case, I don't know it'd matter though. Something to keep in mind, I guess.