What's your favourite format for collecting/listening to music, and why?

Digital, same for movies, games and books... convenience.

As of now its digital. Cause of Spotify reasons.

I used to collect Vinyls but then I stopped

Good topic, my CD collection is great for convenience but I couldn’t replace my Vinyl with them. There’s just something about the huge artwork on the cover, different coloured records, watching my Sony automatic turntable do its magic. Playing Vinyl is so much more involved than just sticking a CD in, you’re a part of the experience.

CDs. They are more compact than vinyl but admittedly they sound nearly identical to digital downloads and streams as they are still digital.
Shoutout to digital as well for convenience and vinyl for being cool. I grew up with some cassettes too but barely have listened to them for over 20 years. I bought a cheap shoebox player last year and have the Guardians Awesome Mixes on cassette.
I've bought more vinyl since I got a record player but haven't given up on CD entirely.

Like with everything else I stick with digital. As a kid I had cassettes and CDs. I had a brick of an MP3 player I got in 2005 or 2006 that I used for several years before I got my first smartphone. 

Whatever best ensures access to music I've purchased in the long run and gives me maximal freedom. In practice, I find that to be CDs. Vinyl might also fit the bill, but I'd need large equipment for that, and I have no idea where I'd store all those vinyl discs. CDs on the other hand are pretty handy.

I'm not a fan of digital, but I could be persuaded otherwise if FLAC (or some other reasonable lossless format) was an option. Sadly it doesn't seem like it's very commonly available, so its usability as an option is limited.

I like the 'warmer' sound of Vinyl but that can be captured on CD as well. Plenty of my techno/trance CDs are recorded from mixing Vinyl with all the Vinyl 'artifacts' present. The main reason for sticking to CDs is, I play them in the car while driving. 

Digital sounds pretty good as well today (early mp3 sucked) but I prefer physical albums and the simplicity of one format/source for all.

The best sound is on blu-ray music though, DTS HD-MA 5.1/7.1 24bit 96/192khz. Yet choice is very limited there and don't have that equipment in the car either. Sounds glorious at home but limited to a few concert videos. 

I do play digital at home as well but find the surround always lacking (same with CD which only does pro logic surround) so I play those at 5 or 7 channel stereo. It's a shame 96khz CD never appeared. Not that you can hear tones above 20khz nor want to, but it does sound more vibrant/clean. (Although that could just be from putting more effort into 96/192 khz sound mixes)

I grew up with Vinyl and cassette tapes, that choice was easy. Cassettes are awful. (Prying the tape out of the player ugh)

I'm no audiophile, but I hate the thought of not having access to 'full' quality. I guess not going vinyl all the way should bother me more, but gotta have some limits, and mine is there. I'm somewhat curious about vinyls though, because I don't actually have any experience worth mentioning with them, but like I said, I find them impractical for my needs. I don't mind digital formats, just digital distribution unless it gives me access to a lossless format. In fact, I've actually ripped my CD collection to lossy Vorbis files (quality 9.0 seems reasonable for my needs), aside from some rared bonus tracks that would be more annoying to get a hold of if something were to happen to my CDs. I just want the lossless option to be there if I want it for any reason (in particular if I ever want to change formats, for any reason).

Vinyl is not 'full' quality. It can capture up to 50 khz and some claim over, while CD maxes out at 44.1 Khz, but Vinyl has a smaller audio range (55-70 dB vs 96 dB for CD) and bass has to be reduced (and often mono bass) on Vinyl to avoid skipping. Deep base groves can make records skip.

The main attraction of Vinyl are the covers, the art work and feel. All the 'warm' sound feelings can be emulated / recorded on digital formats.