Geez - tough choices but for different reasons.
Like the other replies, I love TZ but it confuses the heck out of the non-pinball crowd. Whenever somebody else would play the game I could just see the question marks above their head - I would spend time explaining the basics to no avail. Eventually I got to the point where I could get Lost in the Zone regularly and sold it.
TAF is very fun - simple gameplay and the magnets add an element of randomness. Getting very expensive now.
TSPP is a spectacular game but a little stop and go and game times can get into the 45+ min range. If you enjoy the theme and digging deep into the rules, this is a fun one.
Funhouse is one of my favorites. Can be a quick game depending on how it's setup but keeps me coming back. Great sound and light package and everybody knows what to do - hit Rudy. Hard to find in reasonable condition and price though.
I really don't see any of them losing their value. I would focus more on what style of game you like to play and what would compliment your current lineup.