It was the first bitcoin mining pool and undoubtedly paved the way for many other mining pools.
Founded by Marek Palatinus, the current CEO of Satoshi Labs, it is Czech Republic-based and follows score-based measures to discourage pool-hopping.
This is a medium sized pool. Slushpool demands a 2% fee from each block solution reward. Slushpool's dashboard is very user friendly and provides great details with regular updates.