Recently I've been testing a couple DNS servers to see which one performs the fastest because I need a really fast connection. On to my point, if I were to use a malicious DNS, could they see my passwords and plain text data on HTTP websites? If not, are they capable of doing anything else malicious?
Not directly, but they can collude with other agents to see passwords sent in clear-text.
All that a DNS server is used for is mapping a domain name to an IP address. So DNS servers will never see data you communicated between you and a website. They just aren't involved in that part of the communication.
That said, a DNS server can incorrectly return an IP for a website that is running a phishing or similar scam. You might then enter your credentials into that fake site. This is easy to do as your URL bar will state that you're visiting your intended destination site even though you are visiting a scammer's site.
So, a rogue DNS server cannot steal your passwords, but a rogue DNS server in cahoots with a phishing site can certainly steal your credentials.
If you are using HTTPS and don't blindly click through security logins, HTTPS will ensure that you are talking to the real site and not a phisher.
No, they are unlikely to see your passwords. The DNS sever is queried in the process of converting a domain name (i.e. www.google.com) to an IP address (i.e. a string of numbers), this process is called address translation. The DNS server is not involved in any other part of your Internet access.
Yes, DNS servers can be hacked. In particular, DNS poisoning is a problem where the IP address table of a DNS sever cache becomes filled with wrong IP addresses. For example, your update server's domain name for your operating system might be redirected to the wrong IP address and you might receive tainted updates.