The scary thing about zombies is their creeping inevitability, like the slow descent of old age or the metastasizing Starbucks invasion.
Deanimator emphasized the slow march of death by putting you behind the gun of a never-harried nebbish, standing off an endless army of newly risen silhouettes. The play is simple, with one upgrade (a shotgun) that's ultimately unsatisfying. A cruel mediation on the nature of living?
Boxhead slapped the player into a Lego-meets-Robotron endless killing spree. Unlike Deanimator, the upgrades are plentiful, but ultimately extend the game too far. After, what, the 100th level or so, killing the zombies just lacks any sort of sense of accomplishment. That, and frankly, the shotgun is the best weapon by far, making all the other upgrades largely irrelevant (aside from maybe the fake walls, which are handy for reshaping the field).
The Last Stand deals with the zombie hoard in a broad, "castle defense" sort of way, and largely overcomes the deficits of the other two games— it has enough upgrades that the gameplay is varied, and differing zombies keep the difficulty a bit more in line. It still ultimately becomes too easy (assuming that you get lucky— one complaint is that chance can make the game way too hard or way too easy, depending on how your scavenging runs go), but it has the good sense to end the game. Sure, that undermines the hard determinism that makes zombies thrilling, but it works in terms of fun.