Quality is debatable, but the Halo series has been vastly more influencial. It's one of the biggest reasons that the Xboxes have been as succesful as they have, and has had a huge impact on how console FPS games are played.

Metal Gear Solid might be unique, but that's not the same as influencial. Relatively few games have copied it, and it certainly can't claim to be an important part of the success of the Playstations. One might argue that the genre of stealth games owes a lot to it, but it came virtually simultaneously with Thief, and has since been surpassed in gameplay by a horde of other games.

Casual: Halo
Hardcore: Metal Gear Solid

Both good series, but in terms of influence its got to be Halo. Because of Halo, the FPS now reigns supreme in video games, seemingly a new one being released every other week (much to my dismay).

MGS has a bigger franchise and a better story but Halo is more fun gameplay wise.

Legacy? Capturing the heart and imagination?

Metal Gear Solid. Hands down. No one ever remembers an FPS for it's story, but rather it's multiplayer. And all that is is just killing random people across the internet. MGS....you can't even describe it...it'd be too  convoluted and your head would explode

There were plenty of FPSs on consoles before Halo, it's just that nowadays as long as you stick a gun in someones hand and put it in the first person, the game will sell. It's more guaranteed success than it is attributing to Halo, IMO.

And because there were FPSs on consoles before Halo, I don't see how the genre owes itsself to it.

It has to be Halo for sure. I mean Metal Gear Solid is very influential in its interactive story telling and gameplay but without Halo, the market wouldn't be flooded with top notch FPS games.

Halo also started the online multiplayer aspect for games on consoles since multiplayer was only popular for PCs 10 years ago and now pretty much every game has a multiplayer feature built in the disc now a days.

You'd have to give the majority of that credit to M$ themselves for incorporating broadband connections in the Xbox. If they didn't do that, Halo would've just been another FPS with local multiplayer, which was done way before that. Speaking of which, all this talk about Halo taking over FPS....ahem...Goldeneye?

My vote goes down for Halo
Halo did for consoles what Half-life did for PCs.

halo revolutionised fps on consoles+1