Mine: M&LBIS especially for The Grand Finale, Galaxy 1 and Super Paper Mario (The Ultimate Show goes so hard). But if I had to choose one I would say Galaxy 1, Space Junk, Gusty Garden, Battle Rock, Comet Observatory and more. But what you think?
I always answer Mario Sports Mix, it's always the first answer that pops into my mind whenever I'm asked for my favorite Mario soundtrack. To me, it consistently has the most songs I would listen to percentage-wise out of all Mario soundtracks, and it covers a fairly wide amount of genres. It's a shame this isn't in Nintendo Music and I wouldn't bet on it making it to Nintendo music, it's very niche.
I don't really have a "favourite" soundtrack but here's some that I really like and probably spent way too much time typing stuff up for:
Mario & Luigi: Bowser's Inside Story - Mostly a nostalgia pick, but a Yoko Shimomura soundtrack never misses. The calmness of Cavi Cape, breezy fun of Forever in the Plains and eeriness of The Castle Depths are some of my favourite RPG location tracks ever. Plus they all have (inside) variations that give the songs a slightly different vibe. The battle themes are also very fun, and there's no topping The Grand Finale as possibly the best final boss theme ever. Damn it now I want to play this game again.
Mario Strikers Charged - Maybe a more forgotten one on the list, but probably the first Mario game with live-recorded music and there's so much variety here, from Bowser Jr.'s surf rock theme to Luigi's iconic flamenco theme. The stadium themes themselves are great too, with The Dump and Crystal Canyon having these very distinctive… idk I want to say country rock kind of sound? The Classroom is one of the best training mode themes ever. I also love the sudden death theme and to this day am still sad that the extended version in the intro cinematic isn't available without sfx. Also features the best pause menu theme in history.
Super Mario Galaxy - One of the first Nintendo games to feature an orchestrated soundtrack (I think the first would be Melee?) and stands out for a good reason. Of course you've got well known tracks like Good Egg Galaxy and Gusty Garden, but the ones I really appreciate are some of the ones that use a combination of orchestral and synthetic instruments to create some wholly unique, space-y atmospheric tracks. Buoy Base Galaxy and Space Junk Road are amazing.
Super Mario Galaxy 2 - Maybe a bit of bias talking as I played it before Galaxy 1, but I generally prefer this games' soundtrack to the original. The main theme is much more playful and bombastic, which carries over into Sky Station Galaxy. Yoshi Star Galaxy, Puzzle Plank Galaxy, Melty Monster Galaxy, Thowback Galaxy, Bowser's Galaxy Genereator… there's so many tracks that I love from this game in particular. The different world themes are nice too, something that Galaxy 1 didn't have.
Super Mario 3D World - Similar to the Galaxy games, 3D World features live music but this time with a big band as opposed to an orchestra, which leads to some very funky tunes like Hisstocrat and World Bowser which are fantastic. I won't list examples this time but the level soundtracks are all very consistently solid. Although I will shoutout Shifty Boo Mansion as possibly my favourite ghost house level track out of the entire series.
Mario Kart 8 - If you're noticing a theme with most of these, it's that they've all got music that's live-recorded with real instruments, and they're all amazing. You've all played this game, you know the music's good, I don't need to elaborate here.
Paper Mario: The Origami King - I don't know what happened behind the scenes for the Paper Mario team to suddenly switch into live music with the heaviest metal the series have ever seen but it absolutely rocks. All of the battle themes in this game are awesome, with extra points for having "thinking" variations for when you're standing still long enough. The Shifty Sticker is probably my favourite of the bunch. Special shoutout to Autumn mountain for having the excellent use of Asian instruments.
Paper Mario: The Thousand Year Door (Switch) - I already enjoyed quite a few tracks from the original, so adding in the excellent musicianship from The Origami King just helps elevate it to a new level. There are some tracks that I feel like they missed the mark with when it comes to keeping the vibe of the original, but there's no denying how fun this games soundtrack is. The new extensions to Doopliss' theme or the Cortez battle are excellent, as well as all the new variations for the basic battle theme for each area. The brand new tracks like Atomic Boo and Prince Mush also sound like they were taken straight out of Origami King as well and are worth a listen. The new version Palace of Shadow is awesome and something I regularly revisit. Metal mario underground theme is just cool man.