TL;DR: Check if pool mints blocks, has reasonable parameters and does what you like.
Performance. To receive rewards, a pool has to be minting blocks. Not only the size of a stake matters, but also the robust, scalable, high-available technical setup with failover and redundancy. For now, the Cardano node hardware requirements are rather low: 1-2 cores, 8GB RAM and good network. It is expected, however, to experience increase in the used compute and network traffic in the future, especially following the deployment of smart contracts and further adoption of Cardano.
Decentralisation. The more stake is controlled by a single entity relative to the total stake — the higher is the influence of that entity over the network. At the moment of writing, there are huge actors, such as 1PCT with 29 pools opened or Binance with 62 pools. Consider staking with Single Pool Operators or at least avoiding gigantic enterprises.
Community. Check out the pool’s website and activity. Whether the SPO (Stake Pool Operator) is active and reachable, whether you like the values and contribution of the pool. If you decided to support a smaller pool, that does not mint blocks every epoch, make sure they have a decent marketing strategy and the growth is expected.
Specs. Overall, every performant stake pool will have an average return around 4-6%, thus what matters most is whether you personally feel like supporting this specific pool: would you like more of such pools to exist? However, there will be a small difference in return when looking at the specs. Check out our further explanation on this matter below.
Are you having a big stake and considering joining HYGGE?