Quake II sports some of the best guns around, making other games weapons look like pea-shooters. Still, knowing when to use use what gun may be better than actually having that gun. Try all of the weapons to see which one you like best, I've found that I do faitly well with a Rail Gun.

Weapon	Ammo type/used	When to use
Blaster	None	Weak enemies, or long distance shots requiring a lot of accuracy
Shotgun	1 Shell	Up close on weak to medium enemies
Super Shotgun	2 shells	Up close, or against large groups
Machinegun	1 bullet	Use when you want a continuous cycle of gunfire that doesn't use a lot of ammo
Chaingun	1 bullet	Large and powerful enemies at close to mediun range
Grenade Launcher	1 grenade (can also be thrown by hand)	High or low ledges, around corners
Rocket Launcher	1 rocket	Long range, medium to powerful enemies
Hyper Blaster	1 cell	All purpose
Railgun	1 slug	Powerful enemies, long range, high accuracy
BFG10K	50 cells	Powerful enemies, huge groups.