Hiya.
For me, favorite and best are pretty much the same. For me, I base how "good" an adventure is according to the "replay-factor".
The "replay-factor' is: Can I run this adventure three times, for the same players, and each time have a different story and outcome? If the answere is "No"...it's very poorly designed. If the answere is "Maybe, with some tweeking"...it's badly designed, but still salvageable. If the answere is "Yes, probably, with tweeks"...it's a decent design, but not 'best'. If the answere is "Yes, absolutely."...it's a well designed module.
Now, contrary to what Steel_Wind (above) said, I think The Six-Fold Trial most definitly falls into the "very poorly designed" catagory. There are absolutes and 'single-event reveals' left right and center that are core to the adventure. It may be fun...*once*. Playing it a second time? Why bother... And on the flip side, taking B2, The Keep on the Borderlands, it sits firmly in the "very well designed" catagory. I have had at least 3 full campaigns revolve around the Keep (mostly with the same players)...and each time has been vastly different from the other times. Different story, different sub-plots, different tone, etc. (one campaign centered on the Caves and clearing them out...pretty standard, but tossing in the Cave of the Unknown that connected to the 'caved in' corridor around area 51 and adding a crazed wizard named Mogg the Mad, and we had some nutty underground, dungeon-bashing fun; one involved the evil clerics trying to get powerful enough to assult the keep, while some of the goblins/hobgoblins were trying to do the same thing by recruiting ogres and giants from the surrounding mountains, and the kobolds negotiating with the lizardmen for protection; the last one had a lot of keep-oriented skullduggery and secrets-upon-secrets regarding the nobles, rich merchants, a potential guard-rebellion and the Mad Hermit being at the center of it all).
Anyway, yeah. The best modules ever written, IMHO...
B2 - Keep on the Borderland
I1 - Dwellers of the Forbidden City
L1 - The Secret of Bone Hill
U1 - The Sinister Secret of Saltmarsh
Each of those can be played multiple times with significantly different stories and outcomes. With U1, for example, I've had groups flee the Sea Ghost and warn the town, organizing for the inevitable pirate/sahougin assult (who had the help of the black dragon in the swamp at this point)...I've also had people take over the Sea Ghost and then take the place of the pirates, but selling weapons to both the sahougin *and* the town (basically, they turned into unscrupulous pirate/weapons-dealers). There's no way I have the time or the space to go into how different these and others (especially I1...wow! The variety of potential is astounding in that one!) have been. Without a doubt, these are the BEST modules ever written. Period.
^_^
Paul L. Ming