I don't know what this quote is referring to, but I'm hard-pressed to believe he's referring to Tyrion and Tysha. It's a relatively suitable explanation, I suppose, but I think he's talking about a Stark. Who, and for what reason, I do not know, but the entire conversation leading up to this statement was about Starks.
Jaime is telling Catelyn how Brandon and Rickard die. Catelyn thinks Jaime is trying to claim that he killed Aerys to avenge Brandon Stark, and Jaime says:
"I made no such claim. The Starks are nothing to me. I will say, I find it passing odd that I am loved by one for a kindness I never did, and reviled by so many for my finest act."
When I read this statement, I took him to be saying, "I find it passing odd that I am loved by one [Stark] for a kindness I never did, and reviled by so many [Starks] for my finest act."
I do not know which Stark he is referring to, but I lean most toward Lyanna, followed by Brandon or Rickard. We know Ned and Catelyn don't like him, and other than Bran's vague disquiet, none of the other Starks have any feelings toward him. He's not even on Arya's kill list.
I don't think it's referring to Tyrion and Tysha for the following reasons:
- The statement seems to imply Starks, given the sentence preceding it.
- Tyrion never expresses gratitude for this supposed kindness. In fact, Tyrion is heart-broken to learn the supposed truth of Tysha. Tyrion grows up tormented by his perceived misplaced affection for a whore, and questions his ability to be loved.
- The "loved him for a lie" quote that is often attached to Jaime's quote to lend credibility to the argument doesn't really fit. Tyrion doesn't love Jaime because Jaime lied about Tysha being a whore. Jaime feels guilty about hiding the truth from Tyrion, and Jaime is worried that Tyrion's love for him hinges on the truth about Tysha.