My most previous phone prior to my current Nexus 4 was the Galaxy S3 on both Verizon and AT&T. I also used the iPhone 5 for a bit, and had the OG Galaxy Note (international version) prior to all those.
I don't think I was 100% happy with ANY of those phones. 90% sure. I don't know if it's possible to be 100% happy. There's always something.
I hated the S3 on Verizon, that damn wifi always in the notification shade and i had a big issue with letting anyone else touch my phone. there just wasn't enough negative space, whether people hit the capacitive buttons when trying to take a picture or volume/power while watching videos, it was just really annoying. The Note was too big and clunky. It was very awkward at times. I use my phone while doing other stuff a lot, so it just doesn't work out for me.
I can say that I've decided to try my first ROM on the Nexus and it made an already impressive phone, that much more. Also, from my very little experience rooting/ROM'ing with the S3's I had, tweaking things on the Nexus is MUCH easier. So simple.
However, there are a few things that have me looking towards that HTC ONE. Audio on the Nexus sucks. Watching videos requires you to cup your hand around the back so you can hear it. Those dual amped up front speakers on the HTC ONE make complete sense!
Also, I'm rocking the Nexus naked and the glass has me SO nervous, all the time.
The camera isn't great. Close-up shots are actually very good. But taking regular shots requires a lot of pre-shot tweaking and it's typically a little fuzzy. I'm excited about HTC's new camera, especially since the camera on the ONE X was pretty darn good.
My one "can't live without it" on the Nexus is tethering. I don't pay for a home wireless service, I use my t-mobile prepaid data for my laptop when I'm at home. It's very simple and straightforward, built into the Nexus. I will absolutely root the ONE, so I'm not too concerned with losing that. Plus, in learning about root and rom, you pretty much negate any "pure android" argument because there are plenty of ROMs out there that mimic that pure android experience that can be applied to any rooted phone out there like the S3, Note 2, HTC ONE.