Of all the video games that have performed poorly in sales, which one(s) do you think were least deserving or their failure? (Or in other words, most deserving of selling better)

Zack & Wiki: Quest for Barbaros' Treasure is the first one to come to mind.

Others include Beyond Good & Evil, Okami, and Psychonauts.

Singularity comes to mind.

Suikoden V.

Both Shenmue games. They were some gems created with care and love, and while they had their flaws, ultimately they were a fun, memorable and loveable adventure affair. It is sad that it never got to meet Sega's sales expectations.

This, even though I didn't play Okami (yet!) =P

I was really happy when Psychonauts 2 got funded, that game will be day one to me.

Nier

Ride to Hell: Retribution.

Cause it didn't deserve "weak sales," it didn't deserve to "flop," it deserved to have every copy taken, microwaved, burned, and buried  in a shallow grave at the bottom of the ocean and the creators fined for crimes against common decency.

EarthBound (when it first came out in America)

Disgaea 5 (In Japan at least), Warhammer Age of Reckoning, Resistance 3, Dynasty Tactics 2, a few others I suppose.