The very best guns for a Marshal ship (and really most others…) are called ‘Entropic Disintegrator’ (under ‘precursor weapons’ in the market). Worth every isk.
Not only are they quite powerful, but they actually have an optimal range that goes all the way to it’s maximum range. Yes, they are not the longest-range guns in the game, but their incredibly-high tracking ability, awesome optimal, amazing damage application profile, and other superb attributes outweigh that by far.
Entropic Disintegrators are great against small ships up to any size ship because they are a continuous beam weapon that builds up and does more damage per cycle to the target the longer the beam is on the target! Entropic Disintegrators already have an impressive tracking ability, but if you put a targeting computer on your ship with a targeting speed script in it, most things you shoot it at won’t be able to escape it’s burning, high -damage beam!
Entropic Disintegrators are also good for pvp, but thankfully not that common. Maybe because even though you greatly increase your chances of winning a fight if you have them as your guns, you probably want to put them on a really good ship, not sure though.
Sometimes mean players will try to blow up your ship just to be jerks when you are just minding your own business fighting the NPC pirates but if you shoot Entropic Disintegrators at them they will run away in fear! (Serves them right!)
The regular Entropic Disintegrators are really good…but I’ll let you in to a secret not many players know yet:
Due to recent (awesome) rules change, the faction (Veles Supratidal Entropic Disintegrator) and Officer (Zorya) versions now count as being Tech Level II (T2) guns and can use T2 special ammunition! That is a huge buff and saves a lot of training time! (before one would have to train Entropic Disintegrators all the way up to skill level V even to be able to use T2 types)
So go right now and buy Entropic Disintegrators! Jita is probably the best market to find them available (they’re not terribly common yet)