I just noticed this thread, which - I note - has been running since 2018
My favourite system after Pentax K-mount is... Pentax Q-mount
Does that count?
My Q7 gets almost as much use as my DSLRs, purely because of convenience and light weight, and the fact that the image quality is still easily good enough for many of my use cases. I mainly shoot with the 02 and 06 zooms; occasionally, the 01 prime too. Some my favourite photos I've taken have been with this kit, because I've carried it on days where a DSLR simply wasn't practical.
Aside from Pentax K & Q, I also shoot Sony A and E mount...
My A-mount involvement started with a "vanity" purchase a few years back - a brand new Hasselblad HV, the remaining few of which B&H was selling off at a ridiculous discount from the equally ridiculous original retail price. The HV is a Sony SLT-A99 with better clothes and minor firmware alterations. As such, it uses A-mount lenses (the kit I bought came with the excellent Sony Zeiss 24-70 f/2.8 ZA SSM). Since A-mount is in its final throes, a lot of great glass is available quite inexpensively at times - either when shops are selling off their old stock, or folks are switching systems and selling their lenses on eBay. There are some real bargains to be had if you're patient. As a result, some of the best lenses I own are for A-mount, and the Hasselblad HV is a lovely camera to use them with... not without its quirks, flaws, annoyances and limitations, mind you - but really nice and very capable all the same
My E-mount involvement is purely for utility. I needed a full-frame mirrorless camera with which to shoot my adapted Soviet lenses, including both short-registration rangefinder and longer-registration SLR models. I chose the Sony A7 MkII because it uses the same excellent 24MP sensor as my Hasselblad HV, has in-body image stabilisation, a good EVF (for its age and class), and it was within budget. The fact that I could add the LA-EA4 adapter and shoot my A-mount glass was a huge bonus, and sealed the deal. I don't own a single native E-mount lens... mostly, I shoot this camera with my adapted lenses, and just occasionally I'll use it with an A-mount lens. It's a very decent camera and does exactly what I need, but I've no real love for E-mount other than the adaptability.