Kingdom Hearts main menu theme. It's absolutely a nostalgia hit for me. That game was one of my "get-away" games while in a rough situation. Hearing that music always makes me feel a little safer, like I'm just one step away from a completely different life.
I remember booting it up on the PS3 back in the day and was like "Wha-?" but after playing the game so many times it's definitely become a favorite. Both for how out of place it is but also how perfect it fits after all the context of the game.
Sad they replaced it for the Dark Arisen version but luckily there is a mod for PC that restores it in all its glory. :P
It's weird. It's terrible as music - just random strings with almost no recognizable tune outside of the first few seconds - but it sets a mood better than any other. It's distilled melancholy in music form.
Syndicate was a really niche entry into the Cyberpunk genre that flopped commercially but had some interesting ideas about corp-ocracy and the way businesses of the future would wage warfare against each other like nations used to.
That main menu music has echoed in my brain for years. Deus Ex: Human Revolution would have been first but Syndicate is less well known and I figured someone already mentioned DE:HR at some point here.
I don't remember if it was in the main menu/what the main menu was, but if you walked onto your balcony in the PC game version of Blade Runner, Vangelis' Blade Runner Blues would play, mixed in with the rain and the sounds of the city. I spent a lot of time just standing on that balcony and letting the music play.
The Elden Ring Main Menu theme has always impressed. It sets the stage very well.
If I think of main menu music only one other game cones to my mind and that's Doom Eternal. I'm not sure if it's necessarily a "good" main menu theme, but the whistling definitely stuck with me.
There used to be a mix I would fall asleep to several years ago. It was the engine humming mixed with the occasional beeping and status tones like a computer on the bridge, and the main menu music would play very softly every few minutes. Can't find it anymore, although there are several of those each individually in hour long mixes.
If I want to get pumped though, doom 3 is a good one:https://youtu.be/rs0cSUIKKew?feature=shared although for actual main menu looping music, I'm going to have to agree with GONADS125, battlefield 1942 never fails to get me up and moving. It's a very driving song.
Maybe not my all-time favorite (there are too many to choose from), but I will never forget the first time I launched Rocket League during a Steam free weekend. Back then they had Breathing Underwater as the main theme and the way the logo appeared in sync with the music was such a beautiful sight.
I linked the high quality song above, but if you want to experience exactly what I did, here's a link to a video with the intro captured from the game. Still gives me goosebumps when I see it.