What is the best soundtrack in the series?

Hmm, it's kind of a tough choice. I loved a lot of track Jack Wall did for ME1, all the synth style, and the inclusion of Faunts' "M4 (Part II)" in the end credits. I must have listened to that track hundreds of times after I finished my first run through ME1, and I still listen to it every now and then. I've always wished they'd used that track in the end credits of each game of the original trilogy, sort of like all the Bourne films have used Moby's "Extreme Ways" in the credits. But Jack Wall's soundtrack was amazing. I've been a fan of his since I first heard his work in Myst III. In ME1, it conveyed a lot of emotions to me, from the beautiful emptiness of outer space (the galaxy map theme and the main menu ambiance), to the rushing sense of urgency of the track playing during the drive to the Conduit near the end of the game.

But I also loved ME2's main themes, especially the Suicide Mission theme. I know there are some people who don't like that theme..maybe it's a love it or hate it track, but I love it. Or most of it, anyway.. the intro, used when the start screen first loads up, blew me away. The 'heroic' half of that track is still good, but I'm not overly fond of the lead-in to that part, but it's just a minor complaint; I still love that track. The Normandy Reborn track still gives me chills, and I'm glad they included it in ME3, briefly, near the beginning. Jack Wall's work on ME2 stands out.

ME3 had a lot of good tracks, but I especially liked Clint Mansell's contributions, the solemn piano themes for Leaving Earth make me tear up sometimes when I listen. I've heard that track in a few other, non-videogame related things, or at least one I can name off the top of my head: the preview to the season 2 (I think) finale of Homeland. Perfect use of it there, too..
Sam Hulick really stepped up with "I'm Proud of You" and the romantic "I Was Lost Without You." I also enjoyed Sascha Dikiciyan and Cris Velasco's thrilling Ardat-Yakshi theme, and the music that plays during the reaper chases in the galaxy map.
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ME:A's music doesn't stand out quite as much, but the main theme is nice. The main menu theme (which is, I suppose, the main Andromeda / Initiative theme) is a nice combination of synth and piano. It's understated and ambient, but I think it does a good job of capturing that "beauty of space" thing that the original game's theme's did, just in a bit of a different way. It has a little of ME3's lonely piano feel, but it's a little more..hopeful, I think? I like the galaxy map ambient theme in ME:A, as well.

I guess, overall, I'd say I like ME2's music the most. It's exciting, mysterious, and heroic, all with synthetic instruments. They really pulled it off on that one. ME3 is more cinematic and solemn, bittersweet, but I like that too.

Tough! They're all a little different despite maintaining a similar style throughout the trilogy, and it's mostly a matter of preference. ME1 is my personal favourite, mostly for nostalgia reasons, but it really does a fantastic job of setting the tone for the entire series.

I have select favourites from the tracks of ME2 and 3 (honourable mention to LotSB), but overall ME1 takes the cake. And I also really liked the club tracks from ME2, I think I'm in the minority there though. Andromeda had a few very specific pieces, even specific parts of pieces, which were great, but overall? Forgettable.

My favourites from each game..

ME1 : Liara's world, Uplink, From the Wreckage
ME2 : Samara (The Justicar), As they Enter
ME3 : A Cerberus Agent, A Future for the Krogan, the Ardat-Yakshi

I like 2 to listen to (and in the game).

MEA is not so much of a 'listener' but it does a decent job for ambience.

Me2 of course, Suicide Mission and Reflections

Honestly I'm shocked how much the ME1 soundtrack is loved. Everyone entitled for their opinion of course, but I find that soundtrack to be extremely boring, repetitive generic SciFi crap with no melody what so ever except for Vigil and the Spectre theme. ME2 is the best in-game with how it fits with the gameplay, ME3 is better as a soundtrack to listen to outside the game. ME:A? Like ME1 but with a bit more melody.

Wait, which one of those settings is from the original trilogy?

Hmm, tough one. I think I'll go for 1, with 2 close behind. It's not much of a gap to 3, but Andromeda was sadly lacking, although it had its moments.

ME2 was more consistent than ME1 but ME1 was more memorable (Collector Base aside in 2's favour). There were a couple of problems with 1 that 2 didn't suffer from - I wasn't keen on the constant Normandy music (nothing wrong with the track, just there all the time), or what we got in the endless identical side mission bases, but it had more than enough to rise above that.

I agree and just posted a suggestion in the bugs troubleshooting thread (wasn't sure where else to post) that it would be great if you could hear the music you choose in the pathfinders cabin throughout the whole of the Tempest.

Which is "As They Enter"? Was that one of the DLC soundtracks I don't have?

ME2 by far.

I like the main theme of Andromeda more than the others, but it lacks set piece themes each game in the trilogy possessed for the big moments. ME1 had final assault, ME2 with Jump Drive, Crash Landing and Suicide Mission, and ME3 had the Fleets Arrive and subsequently the Battle for Earth track (sadly absent from the OST).

Samara's track is arguably the loveliest in the OST.