T1+B1+B2

 The ultimate starter module

The Keep from B2 is structurally the same as Hommlet from T1, but without any of the detail. It has the same house of worship, tavern, inn, traders and smiths, and assorted stores. Where none of the NPCs in B2 are detailed, all of the ones in T1 are. T1 also has a small keep in Hommlet, but its under construction and its owners aren't really recognized as the lords of the area. Simply putting all the NPCs from Hommlet into the Keep, or just replacing the Keep wholesale, is basically a straight upgrade. 

B2 is the only one of these modules with a small wilderness region to explore. There is mention of a bandit hideout, lizardmen lairs, and wandering encounters including an old hermit with a pet lion, but these encounters are subject to dice chance and are left to the DM to flesh out. The Moathouse from T1 can easily serve as the bandits' hideout. Simply placing the Moathouse in the marsh in the bandits' "territory" gives them a premade base of operations and a stronger anchor into the campaign. It also comes with Lareth, a Cleric of Chaos, which dovetails nicely with the hidden Temple of Chaos in B2.

 (Also Lareth is apparently implied to be a fallen Paladin)

B1 is assumed to be in the unexplained "Cave of the Unknown" on the B2 wilderness map.  Placing the whole module within in this way grants many more rumors to be given out at the local tavern. And it also creates two more NPC adventurers that "made it", Rohan and Zelligar, along with Rufus and Byrne from T1.