"I feel better."
Translation:Mir geht es besser.
"Mir ist besser." sounds strange to me. I would rather say "Ich fühle mich besser." "Ich bin besser." means that e.g. you have compared to another person a better ability of sth., e.g. calculating: "Ich bin besser im Rechnen als sie." If you talk about state of health, you use "Mir ist..." or "Ich fühle mich...". In the first version, you normally use adjectives in the positive (the version which you find in a dictionary); in the second you can also use comparative (besser/schlechter etc.)
I guess it's because in English "I am better" could mean: 1. I am better (than before/my previous self). or 2. I am (definitively) better in some absolute term(s) (compared to someone/something).
To say 1, German, therefore, skirts the ambiguity by saying: "I feel better than before" = "It is (Es) going (geht) better for me (mir besser)."
To say 2, one would say : "Ich bin besser."
That's just not the way it is said in German. That sounds like, "to me is better" which is a bit nonsensical. The way it is said in German is, "to me it goes better... mir geht es besser". It's a bit like an Australian checking in on a sick friend, "How are you going today, mate? [wie geht es?]" To which the friend would reply, "I'm going better, thanks. [mir geht es besser]"
In English, the friend could omit the word "going" and simply reply, "I am better, thanks" but I don't think that works in German.