I think most writers don't mind how you word it, as long as you comment!

Saying that, something I find nice is when a reader quotes a line that they liked, or mentions a section or scene they really enjoyed.

If you think the characterisation is good, or the dialogue is clever or the plot really works, you can say that too. But even a simple "this was great" or "I really enjoyed this" can go a long way. :)