I would 2nd (or maybe it's 3rd?) Gothic - well, Gothic 3 (I haven't played 1 or 2 yet).
It's one pretty large "seamless" open-world (no "zoning" to anyplace - even into towns, caves, temples and dungeons (which all range is size from smallish to, in some cases, pretty huge).
It has 3 distinct "biomes" (desert, mountains and grasslands, and high mountains). And, while some areas are "gated" by difficult mobs - it isn't really that hard to find ways around them; and thus have the entire landmass (plus several small islands) to explore, trade, hunt, do side-quests, etc.) My favorite is the snow-and-ice covered high-mountains.
It rewards exploration of nooks and crannies (by having stuff to find in them).
Until you start on the "main quest-lines", you don't have to choose any factions either - although you may encounter certain hostilities until you do (after which, you'll then encounter other hostilities - so - it's a wash).
It's one of my favorite open-world RPG's.
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I'd also agree about Stalker (for me, Call of Pripyat in particular) - you can just "live" in a about 2/3's of the world without doing quests (just hunt, find artifacts, trade, etc.). But you do have to progress the main quest a ways in, in order to unlock that last 1/3 - after which you can then again just "live" there. It's also one of the most "alive" worlds I've seen (where the AI's, NPC's and mobs - all do all kinds of things on their own - much of it random).
None of the Stalkers are "seamless" open worlds though - they have "zones"; but the zones are good sized and have tons to do in each one.
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I also found Far Cry 2 to be a very good open world (although divided into two large "halves"). If you like just spending hours exploring (and sneaking) around, FC2 has it. You do eventually run out of things to do unless you eventually pick up the quest lines. But you can get a good number of exploratory hours into it before that happens.
[edit - typos]
Post edited July 01, 2016 by Martek