You can find info like this in the
Spoiler Forum, for example in the thread
All Guns, All Stats - Complete Listing ***Spoiler***. Another source of such info is the FL Datastorm utility which is available somewhere in the download section. Specific details cannot be posted here anyway because they would be spoilers.
However, your question is a bit odd. 'Best Flashpoint' for what? Best in what regard? In most cases the best would be something other than a Flashpoint, even if you restrict the choice to fast-firing weapons with fast projectiles.
To get a grasp on what 'best' could mean we have to look at how guns work in Freelancer: you have a powerplant that has a certain capacity and a certain recharge rate; it is the job of the guns to turn that energy into damage at the target.
As long as the powerplant is charged the limiting factor for damage are the guns, after that the limiting factor is the recharge rate of the powerplant. More powerful guns can realize the damage represented by the powerplant's capacity in a shorter time than less powerful ones can, and this gives you a tactical advantage. Consequently, financial aspects not withstanding the
optimum weapon class for a ship is always the
maximum it can mount and the 'optimum weapon class' shown in the in-game ship info is misleading; it is probably intended for people who don't know what they're doing and want to fire continuously while in battle.
Some weapon types are more energy efficient than others, which means you can do more damage with the same powerplant capacity; just look at the stats.
Another limiting factor is your ability to actually hit the target with your guns; fast-firing weapons are easier than slow-firing ones, and weapons with fast projectiles are easier than ones with slow projectiles. The weapons with the best energy/damage characteristics tend to have slow refire rate and slow projectiles but they are of no use if you cannot hit the enemy with them. So you have to balance your skill into the choice.
Other aspects for weapon choice: different types of weapons have different efficiency against the various kinds of shields (Graviton/Positron/Molecular); the difference between best and worst can be about one 3rd. Availability and price are other concerns that determine what the 'best' gun is at a certain point in the game.
If you want to max out your current ship you may want to look at Xeno guns. They have maximum damage for their class (more than most guns of the next higher class even) and all other characteristics are either good or very good.