It would probably work best as a high-budget (Like...super fucking huge) miniseries, but Final Fantasy 7. I've been replaying the game recently, and while I've only ever played FF8, some 10 and 13, 7 feels like it has the potential to be amazing if handled right. But then again...being handled right is the bane of every video game movie. DooM didn't suck because the acting was slightly above average, it sucked because there was no Hell, one of the main reasons I went to see it.
The Force Unleashed could be rather cool, assuming they got the same guy who plays Starkiller to play himself.
Mass Effect 1 would make a rather cool movie, and you could technically end it without the need for sequels (Killing Sovereign compltely stopped the Reaper invasion instead of delaying it).
I feel obligated to say Halo. No matter what people say about the series, those live-action trailers for Halo 3 were brilliant, and from what we got with District 9, the director could have made an amazing movie. I'm honestly surprised no one seems to want to touch it, thinking it won't be profitable. If every one person out of ten who bought a Halo game went to see the movie...it'd make a tonne of money.
Personally, I'm rather against a Metal Gear Solid movie simply because the games play as interactive movies. I want to see a Halo movie because I want to see MC and UNSC Marines shooting the shit out of Covenant and Flood, since that never happens in a cut scene. I want a Mass Effect film because I want to see some of the scenes you play in an epic cutscene. Same with Force Unleashed (Like bringing down that cruiser thing). I want an FF7 film because I want to see that massive battle at Junon last longer than thirty seconds, and not look like it was made in 1996. But seeing Snake beat up Liquid? That happens in a cutscene of the game. There is a cut scene where you see Grey Fox going insane. Hell, there is a cut scene of Snake fighting a boss before or after every boss fight. There is nothing a film could do which the game hasn't already done. The only thing a film could do would be to change entire sections to make it only two hours long, removing a lot of good content.
But...that's just me.