I've finished them all except for Risen 3 (still getting to that one…) and often multiple times.
1. Gothic
A wonderfully compelling design with a fun narrative and excellent faction structure, creating some truly memorable choices with consequences. I've finished it three times; underlining its classic status. From a coherency of world and role-playing perspective, I think this game is unrivaled in the PB catalogue.
2. Gothic 2+Night of the Raven
Whilst I completed Gothic 2 originally without the expansion first up shortly after playing Morrowind, my first experience with the expansion was played as a mage back in the summer of '07 and it was utterly gripping right to the end. I can see myself replaying this game one day allying with Lee and investigating the archery and stealth possibilities more.
3. Gothic 2 (Vanilla) I joined the paladins in my first play-through. This was also my very first Gothic experience. (I still have my reports written on the old Ironworks forum)
4. Risen
This return to a more classical G2 oriented game whilst slightly by-the numbers, was still highly enjoyable for me. A captivating, mellow and personal soundtrack by Kai Rosencranz helped create some truly memorable moments.
5. Gothic 3 with community patches installed. (I think I last played 1.74?)
I couldn't get over the relative weakness of the melee system compared to its predecessors yet admired the world's breadth and the sheer ambitious scope of the game. I've still managed to complete it twice despite its shortcomings.
Another staggeringly gorgeous soundtrack from KaiRo.
6. Risen 2.
Bawdy, vulgar and full of adventurous pirate spirit, this game was still fun to play yet lacked the persistence and compelling quality of design from Gothic 1, 2 and the first Risen.
Who knows where I'll rank Risen 3? I've only taken a look at the intro and opening fight sequences which I wasn't terribly impressed by from a dialogue and visual perspective. I'm hoping the world will be more open and enjoyable to explore than the scattered feel of the island hopping from the second game.