I would simply say "I agree" or "I agree with you". Wordier alternatives tend to be more formal without adding significant meaning.

Mona: I THINK THAT ENGLISH IS THE BEST LANGUAGE IN THE WORLD. James: Me, too. Alicia: So true! George: Absolutely. Nancy: That goes without saying. Tom: I'm with you. Patricia: Is there any doubt?

Art: As Randy indicates, you can say; "I concur". I find that I express myself as follows to show this idea. "We are in agreement." "You and I are in accord." "You and I have obtained to a harmonious accord." "Our thinking is parallel on this issue." "We are not in disagreement." "Our thinking is the same." "That is an equivalent idea." "We have equal conclusions."

There are an almost infinitive number of ways to express that idea. I agree, I concur, ditto, likewise, same here, the same.