All of them have there merits. The flamer is the best at fighting close quarters. Eugene is great for general purpose.
Spoiler plus, once you do the pitt and convert all your useless (from a big gun user perspective) small guns ammo to 5mm and have 40k rounds like I did ammo is no longer an issue lol.
Vengeance is great being the most powerful and all, but it breaks faster and it is harder to get repair guns and ammo for than Eugene.
The Missile Launcher is good for taking out multiple opponents and/or generators at the same time. Fun when overlooking evergreen mills. The key is you do not need to hit the target with the missile. You just need to hit something near the target.
Also FYI if you use the weapon mods kits mod and put laser sights on eugene and the missile launcher they get to be quite decently accurate. Also it is pretty sweet and BA looking to have a double missile launcher when you use the extended magazine mod.
Also it helps to use the realistic explosions mod that makes the vanilla explosions 1.5 - 2 times larger in radius. it is balanced though because you have to be a little more careful not to blow yourself up, and your enemies weapons are affected too. Also car explosions get bigger as well.