I think the beauty of the PowerBook was ruined when it morphed into the MacBook Pro. The bezel around the screen looks horrible to me now. Too wide on top, too many cut-outs, however necessary.
The PowerMac (Mac Pro) was ruined with the additional optical drive. It's just not the same clean design anymore, especially with the extra ports on the front, although the interior is a marvel.
The iMac still looks great, although if they shave an inch or so off the bottom, that would make it perfect.
The MacBook wins, then. It (the iBook) is the only computer that has gotten more attractive than before, although it did undergo a fairly substantial facelift to be fair.
The Mac mini is probably best of all, though! Perfect minimalist Apple design!