What in your view are the greatest games that were never followed up, what made them great, and do you wish they got sequels, or do you think they work best as a self-contained one-off gem?

Sleeping Dogs... i dont know how to explain why I love this game so much, but... Sleeping Dogs

Eternal Darkness - The sanity system that took the things that can happen almost out of the game and into real life. I think it works well as a stand alone title, but more games that break the third wall in similar ways are welcome. 

Ristar.

Released at the end of Megadrive's life cycle after the Saturn was out (in Japan) but quite honestly, think it was the best game Sonic Team ever made.

It's the best playing TPS ever made. It also ends on a cliffhanger. 

Great picks so far; don't forget the rest of the OP; do you wish they became a series, or do you think they work better standalone?

+1 for Eternal Darkness. I think that could have been a great franchise with potential for an expansive, interesting lore.

Space Station Silicon Valley. A really fun, quirky puzzle platformer for the N64. I'm sure they could've done some cool stuff with future games / more powerful hardware.

Powerslave (Exhumed). Incredible mid 90s first person shooter with an Ancient Egyptian theme. Shares some similar traits with Metroidvanias / Metroid Prime. There was talk of a sequel at the time (which would go from first to third person), so I guess the developers felt they could take things further (not so sure myself, at least from a story perspective).

Wii U in a nutshell for me...

Tokyo Mirage Sessions #FE, LEGO City Undercover, The Wonderful 101, Nintendo Land, Zombi U and Pokken Tournament.

It's a shame to think that Zombi U and Nintendo Land sequels would not be possible to make without heavy gameplay/controls restrictions due to them being dual-screen focused games,

I need more VanQuish. 

I hope Metal Gear Rising counts. Also ended asking for a sequel and we almost got one. We got a teaser for one then nothing.