A few of the ones I've has some pretty good dialogue, my personal favorites are Tennis Ace and Password. Both do an amazing job of building character and make you genuinely like them flaws and all, especially Tennis Ace.
Those were the ones I had in mind too actually. I think it might have to do with how the other characters actively interact with eachother rather than just talking to the protagonist. Allows a lot more options and opportunities to show different sides of characters. Even after Tennis Ace's prologue/Yuuchi route, the dialogue between just Yuuichi and whoever you pick is still pretty good (although I still struggle to play any route other than Shuichi's. Guilt intensifies every time he comes on screen.)
I very much look forward to Password's development. Murder mysteries are my favorite genre and I went three days doing nothing but just playing Password.
I think Repeat also does fairly well, although it IS a bit of a mess. It's not so much consistent as much as it's funny and interesting to me. I love the concept of the wishes and what type of tragic back stories the characters have.