Favorite? B2, Keep on the Borderlands is just classic. It isn't the best module, and a lot of it just doesn't make sense in modern eyes - but for the time it was excellent. Now it is just a huge slice of nostalgia.
Best? For me, it is my starter module. It was designed to be used with players new to my campaign setting but familiar with D&D as a 'tutorial' on my house rules, the way I run combat, and what you can expect out of combat - and as a diagonostic so that I can see how they play. The PCs are given a goal by an NPC, but there are dozens of perfectly reasonable ways for the PCs to tackle the underlying issue. It is best for me because it gives me insight into my new players at the same time it gives them insight into my style as a DM and into how my housrules function. This 'diagnostic' element is something that I don't see discussed in the WotC and 3rd party low level modules, but is a great tool in making your games better suited for your players.