If you can use T2 Hybrid weapons there are only 2-3 types of ammunition that are commonly used and that I would put into my cargo (for PvP). Those are
- (Caldari Navy) Antimatter for short range, high damage with good tracking
- Null for Blasters, Spike for Railguns as long range ammunition (those are T2, can't be used in T1 weapons)
- Void for Blasters as short range ammunition when fighting large ships, as it has bad tracking (also T2 only)
If you can't use T2 weapons, iron and tungsten replace Null/Spike as long range ammunition.
If you're doing missions, stick with regular ammunition. If you are doing PvP you should pay the significantly higher price for the faction munition like Caldary Navy Antimatter.
Here's an overview of the different ammunitions:
Dmg(kin/therm) Cap Range Tracking Falloff
Iron 12 8 -30% 60.0%
Tungsten 16 8 -27% 40.0%
Iridium 16 12 -24% 20.0%
Lead 20 12 -50% 0.0%
Thorium 20 16 -40% -12.5%
Uranium 24 16 -8% -25.0%
Plutonium 24 20 -5% -37.5%
Antimatter 28 20 0% -50.0%
Spike (R) 16 16 0% 80.0% -75%
Javelin (R) 24 32 0% -75.0% 25%
Null (B) 20 24 0% 40.0% -25% 40%
Void (B) 30.8 30.8 0% -25.0% -25% -50%
The two damage values represent the kinetic and the thermal part of the total damage, Cap refers to the capacitor use of the guns.
The data is from the Eve Online quick reference sheet, which contains a few more useful things.