This is funny, I've been meaning to start a game soundtrack thread all this week & just haven't gotten around to it yet. Great minds...
Not sure that I could say what my favorite instrument/sound is because it really depends on the game, my mood, etc. I can name some of my favorite soundtracks tho and why I feel that way.
My all-time favorites are probably Jet Set Radio / Jet Set Radio Future. I keep them in constant rotation on my mp3 player and they not only work great in the game but are also fantastic for listening to in the car. This is not true for all soundtracks.
For example, what I've heard of the Nier soundtrack on its own is excellent, but I had to turn the music off in-game - that overworld music with the really short voice clip that loops WAY too much was maddening.
A recent game soundtrack that I've fallen in love with is the FFXIV soundtrack. I love the music in that game, it's all so gorgeous. It is very pleasant to listen to but is also non-intrusive enough to stand up to looping.
A few other games off the top of my head whose soundtracks have always stood out to me are Final Fantasy VIII, Klonoa: Door to Phantomile, Billy Hatcher (one of the happiest soundtracks in existence - I dare anyone to listen to G.I.A.N.T. E.G.G. without a huge grin on their face!), Maken X, Castlevania: SotN, Super Metroid (some of the best atmospheric music in any game), Phantasy Star Online, Metal Gear Rising (talk about a soundtrack that perfectly fits the game!) and the PSOne Spyro trilogy.
