Master of Orion 3
1 and 2 were leaders in their time, but MOO 3 was bungled badly in production when certain team members left and the result was a game that played poorly and felt incomplete (in fact it was, the team decided after the former primary director left to remove a central element of game play and then patch up the holes it left)

And yes, DX:IW was very disapointing compared to DX, though at least it was a game worth playing in its own right; unfortunately the same cannot be said of MOO 3.