Lately, I was also searching for good poker rooms to play in during this pandemic and lockdown. Thanks, to our members I have found a few goods once.

Here are a few established poker rooms with a very good reputation which are operating for a very long time:

Pokerstars - great poker room with a lot of players. There are almost 80K people by the tables on average.
There is enough traffic, but I don't like or rather for me there are not enough small stakes Texas Holdem tournaments, sit and goes or freerolls, but plenty of Omaha tables and tournaments if this is something for you.

Americas Cardroom and here is also decent traffic. I haven't checked their tournaments lobby closely yet, but I was really happy to see small and medium stakes Texas Holdem tables full of players. There are enough people to play with, literally on all stakes and this is important that there are many fishes. Additionally, they accept deposits and withdrawals in BTC.

Winamax - another great poker room with a lot of players, tournaments, freerolls, bonuses, etc. Only not all countries are allowed and their main focus is on France actually and other not restricted EU countries. Still, everybody is welcome to play and cash out the winnings just need to use the VPN. It is important that they don't have anything against VPNs because it could cause a lot of problems when one will try to withdraw winnings.

I hope this helps a bit. Of course, I only focused on these poker rooms which were not mentioned before in this thread like SwC, Bet online or 888.