How to choose a stake pool?

A pool that is not online when it is supposed to write a block does not get any rewards. So it doesn’t bring much as a staker.

-> Rule: The pool should be online 99.999% of the time & communicate it that way.

A pool that is saturated (=saturated) does not get any more rewards.
Therefore, it does not bring much as a staker.

-> Rule: Saturation should be below 60%.

A pool that does not produce any blocks does not get any rewards.
So it doesn’t bring much as a staker.

-> Rule: A pool should produce at least one block every few epochs.

A pool that has too high costs leaves less rewards for the delegators. 

-> Rule: A pool should charge a maximum fixed fee of 340 ADA & a maximum margin fee of 5%.

This parameter is often underestimated or neglected. Yet it should be the decisive factor once you have found a few pools where the other parameters are correct.

To understand this parameter, one must understand that staking is a kind of lottery in which the stake pools participate every few seconds.

Now this lottery is fair in the long run, but in the short run a pool can be rather lucky or unlucky.

The Lifetime Luck parameter reflects this circumstance. And it does so like this:

So it makes sense, when changing pools or when staking in general, to choose a pool with Lifetime Luck below 100% (provided all other parameters also fit).