it depends what fighters you like. saturn definetly has the edge for perfect CPS2 capcom stuff, darkstalkers series, alpha 1, 2, 3 Gold, the pre MvC vs. games, cyberbots etc., also for example sf alpha 3 is better on saturn because it has the arcade sized sprites.
but dreamcast wins out on sheer fighting game numbers, i don't think any system has more, all the CPS3 stuff, the neo ports, GGX, all the 3D capcom games, vf3, DOA2, Soul Calibur etc.
I'd say for 2D it's a tie between saturn and DC, for 3D clearly DC, although saturn does have some 3D gems like Fighters Megamix which is still awesome.
Xbox is starting to get up there, but there is still less than 8 fighters on the system, the main draw is the online play.