Well... since everyone's doing the "I've owned" thing, I started with the Samsung Focus Flash(WP7) then moved on to the Lumia 520 (gave to my bro), 1020 (gave to my wife), Icon (sold), 1520 (sold), 820 (my daily driver), 830 (sold), 535 (sold), 930 (still got it), then the most recent would be the 635. Even though it's kind of a drag when I have a 930, the fact that it has an SD card puts less restriction on me. Outside of that, I've always liked the Lumia 635's design because they follow the design language of the Microsoft Surface.
I greatly dislike the "rounded rectangle screen island within a brick" thing done with the Lumia 1020, 1520 and 920. The Lumia 820's design is similar, but makes more sense to me because it's just a generally round phone. Lumia Icon/930 looks decent, but it's both too thick and heavy for the time it was released... and honestly we'd expect battery life to be much better on such a thick phone. My 930 with Denim does okay, but back when I had the Icon on Black, it wouldn't even get through a whole day. The 830 feels like what the Icon/930 should've been. Feels much better and it's thinner without compromising on quality... and the battery life was absolutely superb. If not for the Lumia 820 being the only Windows Phone that has an SD card and the ability to play Chaos Rings without the blacked model glitch, I would've never left the 830.
If they ever made a phone that followed the design language of the Surface as well as the 635 without sacrificing the FFC, having a pitiful 512MB of RAM or omitting the SD card slot, that would be my ultimate phone. If we could when get the 1GB version of the 635 on AT&T (with LTE intact), I'd be pleased as punch. With that said, the absolute best design of any Windows Phone isn't a Lumia at all... it actually has one of the most hilariously sales-murdering names of all time: YEZZ Billy 4.7
Even after learning the reason behind the name, it was a dumb thing to name it. The phone itself is slick, smooth, comfortable, and moreover - exposes just how unreasonably thick all other Windows Phones are. If that phone had LTE, a camera that wasn't absolutely god-awful and didn't jip me out of MixRadio, it would be my daily driver no contest.