pokerstars has the most tournaments and a lot of great payout structures. I also like using an HUD on there and taking notes on players to help with strategy.
The problem with Stars is, many of the tournaments have excessively large fields. This makes it difficult to reach the final table, and you dont know in advance, if you will be playing for 1 or 9 hours, which makes it difficult to plan your time.
Also back in 2017 they increased their tournament fee 20% for most limits, which honestly, there is just no way around it, is not great for players. At Stars a 5,5$ MTT is now 4,9$ for the price pool and 0,6$ for the site, while on other sites its still 5,0$ for the price pool and 0,5$ for the site.
Daniel Negreanu famously said, he dont think, players care about these pennies. But well maybe they should. I at least can not see, what make a tournament on Stars worth paying 20% more to play than a similar tournament on for instance 888.