Plus one to what Clipper said.
I like history. Lots of history. The gun has to be one thing. Caliber, lines, action, all working together. They have to make sense. I dunno, it's weird.
I think that Mosins are really gorgeous. Mausers I think are ugly and too machinelike. Late-build SMLEs are pretty. Early ones seem too bogged down with "gadgets" (*cough* volley sight *cough*). Garands are beautiful.
I'm also a big fan of symmetry, or cohesive asymmetry. I think AUGs are ugly. They seem too spacey. Like an overdone laser-gun. I think that AKs are beautiful not for how they look, but because they have attitude. Real big attitude. Desert Eagles are one of the few types of pistols that I think are pretty, mostly because they have a nice symmetry to them. I hate the asymmetric ejection ports on most pistols. For pistols, double-action revolvers are the most beautiful. The cylinder releases on most of them are serious eyesores to me, though. Especially the S&W kind. I think Colt DAs are ugly, because of their releases. Mini-14s would be really pretty if they didn't have that ghastly end to their forestock. Arisakas are beautiful, too. The oft-forgotten Winchester M1895s are gorgeous. Especially the carbines. M1917 Enfields are some of the most beautiful bolt-actions there are. SAFN-49s are good-looking, but not without their eyesores (Magazine, augh!!! It's not even removable on most of them!). Tok rifles are really pretty. Very few subbies qualify for pretty in my book, but the PPSh is really good-looking. And the Sten just works aesthetically, making it beautiful. The P90 is a good-looking gun. I like the Vz. 61 Skorpion. The UZI has a businesslike attitude to it, making it appealing. Thompsons rock. MACs also have that cohesiveness that make them attractive. The G3 is cool, but that stinking cocking handle is a blight to the frame! The M16 and HK416 are pretty, but they have that freaking forward assist that is so ugly to me. I like Mossbergs. I think the Browning BPS Combat Model is probably the most beautiful thing to ever alight on the planet. I like the Atchisson AA-12 concept, but I think the firearm is, quite possibly, the ugliest thing ever conceived.
There.
Now you know what I think is beautiful.