I vote for Curse of the Azure Bonds for nostalgic reasons. It is the first Gold Box game I ever played...The designers get extra points for making the hardest battle in the game completely optional and not connected to any so-called "story line".
I loved that fight! I looked forward to it every time I replayed CAB, which was lots. Another great fight was with the black dragons on Dracandros' tower. Again, it was a very, very tough fight that you could easily avoid. Few other Goldbox games (or UA mods) used that ploy to provide chances for extra fun and challenge.
CAB was also my first Goldbox game. I wonder if nostalgia makes us connect to our first in some special, mystical, goodvibes kinda way. I wrestled with either choosing CAB or DKK. Finally I chose DKK because of the gut-wrenching end to the Sir Karl -- Maya subplot.
Both CAB & DKK share a sense of extra intrique and suspense over many of the other games. They were both more emotionally charged than average, CAB because of the curse on the party, DKK because of the encroaching, even apocalyptic, darkness (a bit like Night of the Living Dead, sometimes). They sustained their tension well, right to their climaxes.
COK's climax failed to match the darker edge of much of its early parts. Too much Kender action.
DQK, likewise, let its story tension grow slack in important places.
POR did quite well, and probably would've been my third pick. Its structure was excellent, but lacked the driving plot dynamic of CAB or DKK.
SSB seemed too confined for me.
POD proved that bigger is not better. The story didn't hold together as well as the earlier ones, and the threat was too big and externalized.
The Savage Frontier games had workable but average plots for the genre.
I liked the Buck Rogers games a lot. I wonder if anyone is going to pick one of them. There was room for improvement overall, but many of the episodes within their storylines were extremely well done. It would've been great to see a Buck Rogers construction kit.