Indeed vanilla Oblivion isn't that pretty, but Knight PT mentioned graphical mods, and some of them improve the quality of oblivion drasticly (i.e. there's a mod out there, making grass look more realistic and at the same time making it use less cpu)

With a few mods, Oblivion can equal the graphical beauty of Witcher EE (and maybe even surpass it)
but as they say in French: les gouts et les couleurs ne se discutent pas!

[Linked Image from miniprofile.xfire.com]