Which is your favorite mission?

Good sugestion!

There is a lot of quality on that list, and both Noveria and Ilos were temping, for different reasons, but I chose Virmire. I often tell my wife (a completely disinterested non-gamer who is likely ignoring me), "This is quite possibly the best two hours of gaming to be had," everytime I airdrop onto Virmire.

I love the airdrop and the drive through the surf. I love the battles with the Geth, disabling the AA guns and opening the gates. I love meeting Kirrahe's team and learning that Saren may actually be on site! And then the tension of splitting the team.

There's the stand-off with Urdnot Wrex, which ends up being a pivotal turning point, not just in the game, but in the entire trilogy.

I love the infiltration as "Shadow", while hearing the other teams over the comms. I love hearing the updates as I help them out by disabling defenses. Fighting all those damn Krogan is pretty sweet. They're everywhere.

Then, finding the beacon and talking to Sovereign! Another pivotal moment.

This is followed closely by a big ass fight, deploying the nuke, finally getting that confrontation with Saren, and losing Ashley (yup, always Ash).

A whole lot of stuff goes down on Virmire. I'm not sure any other part of any other game can really compete with it. Best two hours of gaming to be had.

What Element Zero said! I love pretty much everything about Virmire, even the Mako section, and usually I wasn't too keen on those for the other parts.
Noveria comes in second, but Virmire is really something special for me.

Virmire. It's the most memorable mission of ME1 for me. First playthrough Ashley shot Wrex and my Shep got angry at her about that, then I had to leave her behind to guard the bomb so I could save Alenko. Also it has the first face to face battle with Saren. Really great mission from start to finish, though I don't like driving around with the Mako all that much.

Virmire. I like Kirrahes speech, the little chat with Souvereign, being able to save Wrex so I don't have to deal with Wreav that much in the later games. Oh, and EXPLOSION.

I'm going to have to go with Ilos through the Conduit and into that epic fight on the Citadel. There is something about walking around on Ilos - the music is perfect. An ancient civilization driven into the night, sad mute reminders all over and they still left a gift for us - and a fighting chance. I really did like talking with Vigil because it went so perfectly with the rest of Ilos.

The Citadel battle was also really fun for me, especially seeing Sovereign in the background. I even took shots at it, but this goes along with Ashley's comments about might as well be shooting spitballs.

While I love all those missions, I think I have to choose Feros as my favorite. The set up is really neat, you encounter a truly alien alien, and I love how you are able to save the colony with non-lethal means(I hope this happens more in the new games).

Noveria.
A lot of variables in game.
Great NPCs (as Lorik Quin, Parasinni and Maeko Matsuo).
To be Ripley or not to be Ripley choice.

Initially I went with Ilos, but after some thinking Virmire it is for me. It has the "punchline" for the whole game : meeting Sovereign.
 
It sets the table for what is to come in the next games: "We impose order on the chaos of organic evolution. You exist because we allow it and you will end because we demand it."

Hey, thanks!
To be fair, Hanako did bring up good points in Feros' favor. There were a lot of ways the colony could fare and there's the added hilarity of lifting geth off the bridge and playing biotic volleyball with them.
I have to say Virmire was enjoyable in its scenery. It was gorgeous, the music was fun and the side missions were turning points for your teams.

What if I told you that before my last playthrough I never managed to save him because I always forgot something? Now I know everything and I make my choice at the beginning of the mission and not at the end: The one I don't send with Kirrahe goes BOOM .