This might be putting on the rose-tinted glasses, but for me, TES IV: Oblivion would be the game I'd pick. It was the first quote unquote 'Free will, open world do-whatever-you-want game' I'd ever seen, and I fell in love with it instantly. I remember finishing every side quest in every city and running around looking for ANYTHING that could possibly lead to a secret area. And when that failed, there was always a countless amount of mods available on the internet. I'll never forget that feeling of stepping out from the sewer tutorial, to see the WHOLE OF TAMRIEL spread out in front me.
'I can go....anywhere?'
Gosh. Come to think of it, I think I still have that disk somewhere...