Well, the only Mark I've ever played is a Mark IV. That said, the biggest reason I like it is the flexibility. The Mark I basically only has 1 channel, the Mark II has 2 channels, the Mark III and IV both 3 channels. IV's always have EQ (II's or III's may not), and is basically like having a 4th channel. The IV has quite a few
power amp voicing options well.
The III's (with the proper options) are close to the IV's, but the biggest complaint about them is that all 3 channels share the same EQ. So it's hard to balance the tones and levels of all 3 channels to get the optimal tone overall. With the IV, at least there are more EQ options.
--Brent