There are many styles of dialogue. They vary according to the genre of the novel.
Do you use accents like Mark Twain in Tom Sawyer and Huckleberry Finn, or not bother? Is it authenticity you are after or plain speaking which most readers will understand?
F Scott-Fitzgerald's, use of dialogue in The Great Gatsby is very lifelike for that era. The streams of thought and conversation are brilliantly interspaced with vivid descriptions of movement and light.
The Wide Sargasso Sea by Jean Rhys is a prequel to Jane Eyre by Charlotte Bronte. The dialogue and streams of thought are integral to the setting of life on a tropical island, where the constantly changing light and colours of the fauna are brilliantly portrayed .
The general dialogue and streams of thought bring both books to life. I found myself immersed in those worlds.
In my historical fantasy novel, I am constrained by the way my characters speak anf think. I hope that I have got the balance right. I have tried to keep my dialogue intelligent, but as clear and simple as possible.
Clear and simple is good, I think. Because I write Georgian period I try to give the reader a flavour of 19th century speech without compromising readability or pace too much. At least I hope I do.
What I do find difficult is separating character's speech patterns to individualise them more, particularly because there also class differences to take into account.