Which hybrid ammunitions are best under which circumstances?

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.

All hybrid ammunition uses the same damage types - thermal and kinetic. As a general concept, as the range bonus increases, the damage decreases. As the range bonus decreases ( and goes into negative), the damage increases. Antimatter is the highest raw damage of the T1 hybrid ammunitions, but it also has the lowest range ( with a nice fat -50% range bonus ). Iron has the lowest raw damage, but it has the highest ( +50% ) range.

As for circumstantial usage, don't bother. You will want to stick to one range of ammunition, and fit your ship to reflect that. Same reason you stick with one type of gun, and not mix meta levels on them. With rails, antimatter is fine, because you still have enough range to project DPS out to a fair distance. Plutonium or Uranium, maybe, if your Gunnery skills are a bit low to keep your distance.