My choice for Compose is the right Windows key, which I think I eventually settled on because I use the left one in some keybinds (winkey+s for shell, etc.) and like you, I couldn't part with an alt or ctrl. I've often wondered what other folks tend to use.
To the grandparent: I'm sometimes amused by what Compose defines. There's ∞ (compose + 8 + 8), (compose + # + #), and oddly (compose + C + C + C + P). I think it may depend on system configuration, but I believe libX11 is responsible. (On my system, Arch, the key combinations appear to be documented under /usr/share/doc/libX11/i18n/compose for my locale.)