I vote for Death Song (rationale to follow). (Also, no offense to Berka, I just preferred Death Song to Dungeon Stompage, and I don't particularly believe "rounding out the stories" is an, in any way, valid reason to feature a game).
First off, I would like to state that Death Song and Necromancer were not a part of the same series. They were both set in the same world, true, and they both followed the same story, but they ran during the same timeline and featured entirely different main characters and characters, it is effectively an entirely different story given the major changes to the way it is being showcased. You can say that they are part of the same world, and if it incenses you that two stories are part of the same world and both featured then go ahead and say that, but by definition they are not a series, and the stories they tell are extremely dissimilar.
Aman's statement that Death Song cannot be fully enjoyed without Necromancer is also incorrect. Death Song without Necromancer is just mystery, you never know who the necromancer is or why the world is being destroyed, and that's okay because it's irrelevent; the plight of the main character can be understood without having understood the plight of the villain, Death Song was not about the Necromancer destroying the world but the horror that the people were going through as he did so, and Endmaster portrayed this perfectly.
Death Song far surpassed Dungeon stompage in terms of story, was very non-linear (Yes, there is one "best" ending, but whoever says that the story is linear is clearly ignoring the dozen or so other detailed endings filling the story, just because every ending is not a special one does not make it a linear story), had significantly better built characters and a much more creative follow-through.
Dungeon Stompage's only real merits come from it's excellent use of the advanced editor, the characters themselves were not extraordinarily interesting, the story (as in most games) took a backseat to the actual play, and it made no effort to stray from a very often seen and executed premise - which is fine, because the game was great, the gameplay was fun and the story was still interesting (because a good storyteller can make even the most boring and overused premise fun, and Berka is nothing if not an excellent storyteller) but that does not change the fact that Dungeon Stompage suffered for it.
Death Song as a whole had a far better built world and story, more sympathetic and enjoyable characters; Dungeon Stompage had far superior gameplay, was less "linear" (Which is irrelevent, as being less linear than a non-linear story does not mean much) and made fantastical use of scripting on the site.
The above paragraph is essentially what you're going to hear from everyone who replies to this (save perhaps the comments about linearity), because it is rather clear and obvious (and true).
In the end, it comes up to whether you believe the game or the story is more important, and I have always held the belief that the story is more important than the game, so you have my vote.