They're all increasing in value, but the Mondial and the 348 probably represent the first rung on the Ferrari ladder now, the position previously held by the 308 GT4.
The 308 GT4 increased in value by around 40% last year, so they're finally starting to be appreciated as the fine cars that they are and the sharp Bertone '70's styling is coming back into fashion again.
People who know Ferraris claim that the 308 GT4 is one of the best handling Ferraris there is after the F40. It has a slightly longer wheelbase than the GTB/GTS due to the 2+2 seating and it also has a decent boot/trunk behind the engine, so it's a great weekend car. And of course, it makes a glorious noise.
Paul