Completely disagree on revolver vs pistol, but that's really just personal preference.
SMGs are a good choice. Assault rifles and machines guns I like, but others seem to prefer skipping those, just using an SMG for any rapid fire needs and just using the rocket launcher or sniper for their 2 handed weapon.
Without a doubt, the first thing I'd max out is the SMG. SMGs are very common to find, so ammo isn't a big problem compared to a lot of other weapons. Very deadly when dual wielded as well and the auto aim area is quite generous, especially at maximum power.
For two handed weapons, the Assault rifle is great, what it lacks in ability over the two handed Machine Gun, it makes up in how common it is to find ammo which is a lot more important in my experience.
Between Grenades and Triggered Explosives, I personally prefer the later, but grenades are much more common, so unless you're okay with buying explosives or have a good place to restock, I'd recommend Grenades.