In terms of sheer amount of content, and wow factor, Skyrim. Gameplay and transition between menus is almost seamless, and the graphics are top notch. Also has a very active modding community. Have you looked at the Fallout series at all? IMO it's a more creative series than TES. Both are Bethesda made, and like I said before, have an active modding community. But no matter which way you go (Oblvion/Skyrim/Fallout), you will not be disappointed.