The problem with that, Paul, is that I really don't know which has been there the longest. Of those I mentioned, probably the Spenser. But I read more based on what I'm currently enjoying/interested in. Items get in my TBR because I think they might be interesting, or I think I should read them. I have times when I read more serious stuff, other times when Tinker is just about as serious as I can handle.
Right now, I am in the "keep it light and fluffy" part of the cycle.