[Rupert]

Well, I use High-Tech and Ultra-Tech all the time,so they'd up there if amount of use was the sole criteria. However, while it's very useful I don't read UT for fun that much, so it's off the list. I do read HT for fun and inspiration, so it stay on.

I absolutely love making spaceships with the Spaceships series. I'd say the first of them is my all-time favourite book.

My third favourite these days... hard choice. Bio-Tech probably. I don't directly use it that much, but I find it inspirational any time I'm looking to make some new species or non-standard character.

Back in the 3e days it'd have been Vehicles 2e, Bio-Tech, and no clear third.