In terms of aesthetic, I love Windows 7-era Aero glass design (and to a lesser extent, Vista), but I also love the classic Windows 9x/2000 look. If I had to choose between the two, I'd go with Aero simply because of the start menu search bar.

In both Windows 7 and Vista you could change the theme engine to Windows Classic, which gave you the Windows 9x/2000 look AND the start menu search bar.