I've basically only used Mullvad, but it's often disconnecting. I forgot the exact error, but when I searched for it, I wasn't the only one with that problem.
Also, although it's been a while since I used it on
my spare laptop, it disconnected up to the point where I couldn't get the laptop online again even without Mullvad and eventually just reinstalled my laptop.
I installed Mullvad again yesterday. I didn't have any disconnection issues so far. I paid with Monero. I tried both WireGuard and OpenVPN. As far as settings are concerned, I 've only disabled trackers & ads. So far so good.
Next month I will install IVPN and I will try the same, to see if it suits me better.