Lots of different guns to me fall in the "prettiest gun" cagegory.
Lever rifles - not going to lie, the 1873 is my favorite lever rifle. And the modern Uberti rifles with the full octagon 24" barrels, color case hardened receivers and good wood on them are just beautiful guns.
Single Shot - Sharps 1874, and it's not even close. 30" octagonal barrel, color case hardened receiver, blued finish.
Bolt Action - a good Winchester 70, with a deep blue finish and good wood is artwork to me.
Shotgun - SxS - I love the looks of a good side by side shotgun, especially with damascus barrels. My preference on them is to have exposed hammers though - just something about the exposed hammers that really sets off the look of a good side by side. Pretty wood is obviously a must
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Shotgun - O/U - I like elegance with simplicity, which is why I honestly love the look of the Red Label above most others. The simple satin finish on the receiver with just a hint of scrollwork, nice blued finish on the barrels and pretty wood is just hard to beat for me.
Revolvers (DA) - S&W Model 29 with the old school deep blue that turned the gun into artwork is hard to beat for me.
Revolvers (SA) - Colt Peacemaker, case hardened receiver, 4-5/8" barrel with upgraded, but simple, rosewood grips is absolutely gorgeous
Revolvers (percussion) - Colt 1851 Navy is about the definition of a classic 19th century firearm, and the forefather of most everything modern to boot. (edited)
Pistol (semi auto) - 1911, deep blue finish, minimal roll marks, no front serrations, no rail, blade sights, full size or commander slide and a classic set of rosewood or really nice pecan grips and I'm in love.
I also love a high quality percussion or flintlock rifle as well - just so much craftsmanship and wood on them that when done right, they are a thing of beauty.
Ugliest - most any modern military firearm, including AR's, AK, all bullpup designs, the overwhelming majority of modern semi auto pistols, anything "tactical", most big bore Marlin and Henry lever actions, hi-point anything, glock anything, most things ceracoated or hydro dipped or otherwise painted, most nickel or stainless finish guns (personal preference, I don't like them). Most things in this category are beautifully functional, but flat ugly.