I pretty much just use the MACO MkXII shield on all my fed and the Mk XII KHG/Adapted MACO on my KDF. Pretty much the best shields you got. Some of the Elite Fleet Shields are decent and you can use to augment your resists..etc for PvP. But other than that, the rest are pretty much junk compared the MACO/Adapted MACO/KHG sets.
I can tank an Escort with a Regen Shield I cannot tank an Escort in any other shield. So how can they be useless?
Alright... Let us forget about the Set Shields for a moment... Obviously the best option here is the MACO or Adapted Honorguard unless you can handle super regen from the full assimilated set...
It actually goes a bit like this...
Covaraint shields are HUGE on cap and low on regen... Effectively what this means is that you cannot count on them to heal themselves at all. Moreover Emergency Power to Shields will not fix a Covaraint once it is depleted because extra shield power will not increase the Regen enough and the amount healed is too small. You basically NEED a powerful Transfer Shield Strength or Extend Shields + something else to get it back up and running OR a Reverse Shield Polarity. This means Covaraints are somewhat useless because most things in the game shred shields like they were not there regardless of how much Cap they have.
The places where these shields are good is basically on any ship with POWERFUL shield heals that can reliably escape battle for a few moments to heal them. However, Battle Cloakers do not apply well in this case.
Resilient Shields cut off some of the damage they would receive and preserve your hull. They have less Cap than the Covaraint and less Regen than the Regenerative. However, these regenerate enough to take advantage of shield heals like Emergency Power to. Also, they protect your Hull far better meaning that on any ship with a weak hull these can be pretty ideal. Their cap is not so huge as to be unfillable by a powerful Emergency Power to or Transfer in fairly quick step. This means they can be reliably used throughout a fight.
These are particularly good on Science ships and Escorts who need the extra toughness and something they can up and heal on the fly.
Regenerative shields are an odd brand BUT rather useful under the right circumstances. Their high regeneration and low cap means that Emergency power to shields will heal them right up and make them regen extremely fast. Likewise Transfer will see them back to full rapidly. They also will self heal quickly giving you a thin barrier against kinetic damage between heals. Obviously they disappear fast but so does everything else and being able to come back the fastest generally makes them more useful.
These really shine on Cruisers who can Hull Tank and use these as easily repaired Resistance Granting Barriers. They also shine brightly on Battle Cloaking ships who are not going to even have their shields up long enough to get a good heal from most skills so need their shields to pop fully back up in the blink of an eye. One and Done is the way for a BC user.
Standard shields on the other hand work alright for everything but are not super good on anything... As their name implies. They can ALMOST be better than Regenerative shields though with the right setups due to having more CAP and Slightly less Regen.
However, IMHO, the shield system for the game is fairly broken. These days with every Borg and even a fair number of other NPC"s toting around Instant Shield killing abilities that make it impossible to keep your Covariant's Cap full there is very little importance on how much Cap you have and a lot of importance on how fast you can get shields back because they are going to be popping on and off like Christmas lights anyway.
That is literally the worst possible choice for a cruiser. You WANT that extra bleedthrough on a cruiser because cruisers have the largest HP regen in the game by a MASSIVELY huge margin. As long as your shields are up the 10% bleedthrough gets healed fast enough so your HP pretty much stays at 100%, occasionally dipping to 90%.
If you're using the Jem'Hadar, MACO or Adapted KHG shields, your shields have 10% energy resistance. Now add in 35% resistance for being at 125 shield power. Now add +30% resist for running EPtS III. Hmn. You will appreciate that puts your shield resistance at the cap: 75%. How much better is a shield with 75% resistance than one with 65% resistance? Well, let me show you:
You have two ships: Both have 5000 shield points per facing. One has resistance of 65%, the other has a resistance of 75%. What's the total amount of damage each shield can absorb? Well, the 65% ship is pretty good, soaking up 14,286 damage. The 75% ship, however, can absorb 20,000 damage. That's a 40% increase in punishment absorbed before the breaking point. Which is MORE than the difference in capacity between the Resilient Shield and the Covariant Shield (about 16% more for Covariant), and since the points you regenerate are also worth more for the regenerative shield, also more than the regeneration benefit between the Resilient and Regenerative shield (about 32% for Regenerative).
Now if there's a Regenerative Shield which has the 10% to all energy resistance feature, I will happily equip it. But I don't know of any such shield, and I DO know about the Jem'Hadar Resilient Shield, which is one rather easy quest line away for a Mk XI shield.