The HL2 OST is probably the most understated one on the list but its also the one with the most impact. The album is nothing exceptional as a standalone but when combined with the game's carefully structured set pieces, the result is explosive. Whether you're being chased by the combine fuzz or trying to dodge a killer helicopter, Kelly Bailey's captures the moment perfectly. Among many things, Valve is also a master at pacing and rationing all good things so you spend most of the game being haunted by the silence or sounds of screaming zombies and venomous headcrabs. But when the soundtrack kicks in, oh man.... 
Memorable Tracks: CP Violation, Ravenholm Reprise

The soundtrack for UT 99 was just as hectic and fast paced as the game it complimented. Just like the maps and the different gameplay modes,the music had a lot of variation. From the spacey techno mashups to the racey on the edge rock themes, each track went perfectly with the corresponding map and set you up perfectly for the mayhem about to commence. The fact that most of these original themes are still used is a testament to their musical quality and remains my favorite to this day.  
Memorable Tracks: Mechanism 8, Go Down

This game is goddamn charming. And so is the music that has a very 8-bit era vibe to it. Minimalistic and catchy tunes. The soundtrack makes some interesting use of animal voices, jazz, samba and techno - sometimes all in the same track.  
Memorable Tracks: Sunbaked Savanna

The moment you enter Tamriel, you're treated to the one of the most memorable and grandest opening themes. Sadly, the rest of the game's tracks don't surprise you same way but they all compliment the doom and gloom feeling underlying your adventures in Morrowind. I remember being genuinely spooked by some of the set pieces in countryside and large part of that was due to the music. 
Memorable Track: Nerevar Rising Reprise

A close fifth behind Morrowind. The soundtrack shares a lot of its themes with ES3 with a similar but still distinct adventurous feel.I found the Arcanum track to be really moody in places and that set a good tone for the events in the game. It gets some heavy usage out of string instruments and thats a big plus in my book. Very underrated, much like the game it supports. 
Memorable Track: Main Theme

Dramatic and overstated, the music matches upto the game in every way. It carries the industrial steampunk style of Nosgoth well and compliments the many tragic/ironic moments in the game. Raziel is angry and out for vengeance and you can sense it in the game's music.  
Memorable Tracks: Raziel's Theme, Necropolis (Melchiah's theme)

The opening credits for this game used to be my ringtone for a while. The groovy 60's soundtrack sets the mood perfectly with some really goofy themes to go along with the wacky adventures of the Cate Archer. The retro space age pop used has a very light hearted self conscious feel to it and never gets overbearing. 
Memorable Tracks: Opening credits, No one grooves forever (from the OST)

I know licensed soundtracks are a bit like cheating but I've never heard a better compilation of time pieces like Vice City. Each radio station had a unique color and flavor to it. From the crazy shenanigans of Dj Lazlow to the creepy stereotypical pimp Fernando Martinez, Rockstar really delivers on the humor front. With varied channels like V-Rock and Wave 103, Vice City has you covered no matter what genre of music you like.  
Memorable Tracks: Peace Sells - Megadeth, Crockett's Theme - Jan Hammer

Chanting monks and huge orchestral pieces with a hint of rock sum up Martin O'Donnell's score pretty well. I'm sure most gamers must have heard this theme a million times by now so I don't really need to justify why this is one of my favorites. 
Memorable Tracks: Under cover of night, Covenant dance

I love BioWare's style of storytelling. And Jack Wall knows how to score those character defining moments (He did some themes for the Myst and Splinter Cell series). His work here was obviously inspired by Vangelis' Blade Runner score. Very space noir, if thats such a thing even. 
Memorable Tracks: Battle at Eden Prime, Virmire Ride

Its all about the atmosphere here. Irrational created a scary world and the score just amplifies that sense of fear. It delivers a certain sense of wonder and dread as you explore Rapture and keeps you on the edge constantly. It gets some heavy use out of instruments you usually don't hear in a video game - piano solos, violin crescendos. 
Memorable Tracks: Welcome to Rapture

The music in Warcraft 3 soars and ebbs like the characters in the game. Its mostly orchestral pieces but they are all varied enough that you never really find them to be repetitive. I still listen to the soundtrack during gameplay after nearly 7 years of play. Blizzard's music is just as addictive as their games. I was initially thinking of picking WoW but then I realized that most of my favorite themes (bar the Stormwind City Intro) were picked up directly from W3.
  Memorable Tracks: Blackrock and roll, Reign of Chaos

A typically great Harry Gregson-William soundtrack to go along with Kojima's vision. The string setup during the opening sequence struck a chord with me and that feeling stayed throughout the game. HGW nailed every action sequence with this hollywood style score which is closer to a Michael Bay movie than a video game.
Memorable Track: Old snake

I must say Contra is here purely on nostalgic value. It was my first ever game and the tune has stayed with me. More so than even SMB. The high tempo beats serve as a great background to the sound of all the crazy alien weapon firing sounds in the game. 
Memorable Tracks: Jungle Jam

Another obvious on the list. Who doesn't dig this remixed masterpiece ? Simon Viklund did the impossible and made the old Bionic Commando theme better! I don't remember ever spending so much time browsing a game's menu because of the soundtrack but thats exactly what I did here. I would just hang out in the menu or the starting area of a stage to listen to the awesome chiptunes. 
Memorable Tracks: Heat Wave, BCR Menu Theme

Honestly, its hard to pick one Mega Man from the amazing lineup. Ultimately, I found that most of my favorites were from the 2nd Mega Man, not surprising since its the first one I played. I don't know, maybe you like the techno remixes of Mega Man X. I prefer the pounding themes from MM2 to get my adrenaline going. 
Memorable Tracks: Title theme

Until Arkham Asylum came along, this iteration was the only memorable entry in the franchise. It features a weird mix of the typical dark,gloomy Batman undertones and some ridiculously upbeat themes which sound like they were picked up from the Adam West show. 
  Memorable Tracks: Streets of Desolation

I loved the happy and uplifting music used here. The ambient feel of the music was a perfect fit with the light hearted tone of Jade's world. The gameplay pace was slow and steady and the themes was just as such. 
Memorable Tracks: Home Sweet Home, Peace

Actually, I haven't played this one for quite sometime now but I do remember enjoying the tech noir style of music used in the game a great deal. It had a cyberpunk feel to it which is unique so props for making that work in a game. 
 Memorable Track: Prologue

You can't think of video game music and not be reminded of this Konji Kondo classic. I wasn't going to include this one because its just so obvious. The one that started it all. Its like rating Charlie Chan against modern hollywood movies. Oh well, I caved in and here it is at 20. 
 
Memorable Tracks: The entire soundtrack ? Overworld is probably the one everyone remembers.