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 it.

I stumbled upon this thread on Lowendtalk. Note that I wouldn't trust that VPN, comments say it's not encrypted and basically only meant to avoid the Great Firewall of China, but I like the price model: €1/TB, unlimited time. Especially for light users, paying per TB could be a better deal than unlimited bandwidth while you pay per month. But I haven't seen any other VPN provider offer that.