Just in terms of scale of the world I would have to say Morrowind GOTY edition done it for me, just because of the OG Xbox having basically a mid range PC in a box it allowed a large open world game come straight into the console space and played it quite well. I know the PS2 had some open world games but I don't think any on the same scale as Morrowind in terms of going from picking tiny mushrooms and herbs to brew potions to fighting giant monster squid.
Why not check me out on youtube and help me on the way to 2k subs over at www.youtube.com/stormcloudlive








