Tough to say which is best, it depends on the type of combat.
FDS: Excellent all around ship capable of taking any combat situation in its stride whilst carrying out whatever task you throw at it.
It's as cheap as chips and tough as nails. A very underrated and often overlooked gem.
FAS: Top tier combat ship for both PVE and PVP. Fast and agile, in your face, killer. Probably the best choice for pure combat and bounty hunting.
FGS: " Federal Funship". Seven hard points, SLF, rock hard armor, ridiculously fast acceleration and deceleration all backed up with a size 7 PD. I think that sums up its combat ability. Awesome CZ sweeper and a real fun missile boat.
Might want to start with the FDS as you can outfit it well within that budget and work up from there as the modules are mostly the same.