1. Quake - third was best anyway...
2. The Elder Scrolls - third was best anyway...
3. Heroes of might & magic - third was best anyway...
4. Gothic - for a change, third was the worst until Arcania came about.
Skipped games which do not have actual "series" (Freelancer, Jade Empire) or spoiled entirety of it for me this way or another (Mass Effect most notably).