Tank is by far the most situational part of ship fitting. The big question is: What are you going to do with the ship?

PVE: "predictable" steady damage.

Shield would either be active (Shield Booster) with resistance (Shield Hardeners, Resistance Amps) and enough capacitor modules to be able to run the booster often enough. Capacitor management is key as it can be very hard to have enough capacitor to run the shield booster continuously. As the shield dip, they provide some extra recharge. As such it's good to pulse the booster when shield hits 25% or so.

Shield can also be passive with shield extenders, resistance, Core Defence Shield Purger rigs and Shield Power Relays. This kind of tank works best on battlecruisers. It's a bit unnerving, but you reach your maximum tank when the shield is at 30% or so. If it stays above you're doing great, if it dips below it's time to bail.

Armor will always be active (Armor Repairer). It's key to maximize armor resistance (Armor Hardeners) as the armor repairers are pretty slow acting. Normally, you should be able to permanently run 1 armor repairer or so in PVE. Two can be useful. There is no passive regeneration of armor so the repairer is the only thing keeping you alive.

PVP: Either you have a specific scenario in mind or you assume overwhelming DPS.

If you don't know what you'll face, a buffer is best. Either armor plate(s) + resistance or shield extenders + resistance. Rigs (shield resist, shield extender, armor resistance pumps or trimark armor pumps mostly) can boost the tank further. Unlike active tanking, there is no point where incoming damage overwhelms the repair rate.

Active tanks in PvP tend to be very specialized. The need for speed and tackle module means that the capacitor will be provided by cap booster in anything larger then a frigate. This in turns will use up cargo room.

Let's consider two hypothetical ships, Brick a buffer tanked battlecruiser with 75000 effective hit point (ehp) and Boost an active tanked battlecruiser with 35000 ehp and a 500 dps (3 minutes of cap charges) tank.

• With 500 incoming dps, Brick lasts 150 seconds and Boost lasts 250 seconds.
• With 1000 incoming dps, Brick lasts 75 seconds and Boost lasts 70 seconds.
• With 2000 incoming dps, Brick lasts 37.5 seconds and Boost lasts 23.3 seconds.
• With 4000 incoming dps, Brick lasts 18.75 seconds and Boost lasts 10 seconds.

4000 dps sounds crazy but it's basically the equivalent of a large-ish 0.0 gate camp and 18.75 seconds might be enough to crawl back to the gate. 500 dps is the output of a good skilled T2-fit battlecruiser. So in anything greater then a 1v1, buffer is usually better.