I don't really have a favorite theme, I really like them all if they are done well in the map. I can name a few maps that stand out to me though.
I just love the jungle feel in Borneo. It's also probably my favorite maps layout wise, and I have a lot of nostalgia of playing it from Gun Mettle. The jungle theme always reminds me of a rainy summer day, which I like. Overgrown also has this effect on me.
Powerhouse's detailing reminds me a lot of my memories of play Half Life and playing Black Mesa as soon as it got on steam, so it's also a nostalgic factor for me really. I also just really love the last, and how high the ceilings are, and it's big windows that allow you to look into the other team's last.
I've always looked at Upward as one of Valve's best detailed maps. It just seems like everywhere you go, there's always beams holding up the walls, and there's so many small details to look at. I just look at it and think of the map taking the longest to make out of any Valve-made maps.
I always loved the modern Greek detailing in Illios (Overwatch), and Santorini (CsGo), and I've always been a big fan of modern greek architecture. A couple months ago I wanted to make a Greek map, and then Bak made Eros. It sucks that some people see the map as a rip off from Overwatch, but either way the detailing is done very well, and really captures what I was thinking my Greek idea would be like. The Greek theme is still something I want to do, along with something I think other's should tryout as well.