What is the best soundtrack in the series?

Both.

Normandy Reborn, Suicide Mission; Wall at his finest.

I cannot even think of anything memorable from MEA.

I'm going to have to go with ME1 then.  I just thought that game was excellently scored.  The first scene when you're walking through the Normandy to the theme before you go through the relay for the first time.  The hold the hold the line speech, when Saren kills himself and falls through the glass, rescuing the Destiny Ascension, coming out of the rubble. That somber music playing after Virmire.  I can still hear it all.  Was just great all around.

ME2 was close because I think ME2 had by far the best battle themes, especially in it's DLCs.  The battle themes in Overlord, LOTSB, Stolen Memory were awesome.  Overlord is probably my favorite battle theme in the series.

ME3 - The first scene leaving Earth was powerful for me, but I can't really think of others that stood out in the game.

ME:A - I still think my game was bugged or something because I hardly ever heard any music.  Even in the videos I captured, there's no music or it's really faint drowned out by the other sounds.  I've listened to some tracks, but if we're counting how the game was scored, I don't have much to go off of.

ME2

Seeing the Cerberus SR2 for the first time. Going through the omega 4 relay. The suicide mission.

ME1 is by far my favorite. It nails better than anyone else that retro sci-fi feeling that the original ME was aiming too. I still love to hear Noveria when I want to relax. I still remember the chills from Ilos music while exploring the Prothean ruins and learning the truth behind it. That soundtrack IMO reached a peak that no other ME game did. The later ME2 and ME3 took and epic-orchestral focus I never really liked.

Honestly, to be fair to all the games I'd have to give each game as long as it took ME1's music to grow on me. It was NOT an instant thing by a long shot. I respected the 80's vibe but I didn't think it really achieved greatness. Even now it's hit and miss.

So there's not a favorite soundtrack by game. There's just favorite tracks spread out through the series... very much so including Andromeda.

Hands down ME1. Perfect vibe and atmosphere.

ME2&3 were too generic modern Hollywood blockbuster.

ME:A is almost non-existent.

That's a tough one...

ME1's is great. Vigil in particular stands out. ME2's sound track is good but... too video gamey for the most part? Suicide Mission theme is pure greatness however. MEA, while I like the game, has a bad soundtrack. They really dropped the ball and besides the music at the very start of a new game being good, the rest isn't.

ME3 is, to me, the pinnacle of video game soundtracks, which is a feat because Metal Gear Solid 1, 3 and 4 are amazing tracks as well. The cinematic feel of the music in ME3 is high quality to me. Everything from Leaving Earth to Future for the Krogan, from Reaper Chase to The Catalyst, The Fleets Arrive to I'm Sorry, and finally I'm Proud of You to the wonderfully poignant An End Once and For All. ME3, whatever your opinion on the game might be, has a masterpiece soundtrack that I still will listen to five years later and still feel all the same emotions I felt the first time I played the game through.

ME1, i loved the synth based style they went for as i find it to suit a scifi nicely rather than the orchestral type future games went for.

ME2 is the clear winner to me. Going through the entirety of ME1's soundtrack, there's honestly only two tracks that I enjoy and consider truly iconic, and that's the main theme and Sovereign's theme. Everything else is eh.

One thing ME3 had right, from the start was the OST\music. It has plenty of memorable tracks which sound awesome 'as is' and they are fitting for the moment as you play. However, we should keep in mind that great share of ME3 musical awesomeness is based on smart rearrangements and 'quotes' from ME1.
ME1 themes - love many of them too, but I think that ME3 paid greater attention to its OST and it's...Smarter? I'm not sure how to put it.

ME2 had SM\final run + Normandy reborn. And that's all I can think of (+ Afterlife, yeah).

MEA...well, it's been said multiple times already. MEA has a good ambience, but bad OST, which is heavily lacking on memorable 'iconic' tunes. Had they cut 1\3 of the 'better beginning', starting it right from the 'epic' part - it could have worked. Maybe. As is - no one ever hears it past the 'slow' part.