Enlightenment would be the best looking but not very functional. AmigaOS 3.x with its flat look is appealing but that's more nostalgia.
3.9 with afaos/zune/rtg is amazing. Ambient I can't stand, OS4.1 update 6 gorgeous, feels and looks like a modern OS. 4.0 was terrible.
Windows 8.0/8.1 is good looking too, love the live tiles. Also like the theme updates in 8.1.
Windows Vista felt rushed and not complete. Just look at the half ass loading screen. Win 7 fantastic. Basically how Vista should have been.
I like windows XP in classic look but can't stand the fisher price theme.
Windows 95/98 not a big fan. 3.11 was amazing but that could be nostalgia kicking in again.
BEOS never liked the colors they used.
Of course non GUI wise I would have to rate Linuxs shell with color ansi the best.
There was also a program called gt ripple under Linux that would make the bottom of the screen appear like a pool/wave. Add that to enlightenment drool...
