This is actually a tough one for me! Despite endlessly praising Gothic 2 in my recent retrospective, Gothic 1 left a very strong impression on me.
Things in Gothic 1 I *really* loved, were things like - starting as a weak nobody in a prison colony, and having to scavenge and survive every second in this harsh new land. The various camps were great to explore, and I have a very special place in my heart for the Sect camp. The world was expertly crafted and it really felt like a real place. I could go on and on.
But Gothic 2 wins this one for me, simply because the world design of G2 is really unmatched by most RPGs since. The open-world is just expertly designed, and all the elements feel natural and have an organic feel that lends to a deep sense of world immersion and a feeling that you are exploring a real place. Gothic 1 has a ton of this as well, but I feel like I prefer the open-world of the second Gothic game just a bit more over the world of the first game. It is close, though, for sure.
So, rolling with G2 as my favorite, G1 as a close second, and then Risen and Risen 2. Those are the only 4 PB games I've played thus far.
Long live Gothic! (and PB!!)