I think we need some goal posts here to define what we mean by great. If we mean longest then you would probably look to the likes of Molly Blooms soliloquy in Ulyssess, but that doesn't sound right does it? Longest is not what we are after. Some suggestions like nowaypablos Vonnegut: "so it goes". While I like the Slaughter House 5 and how this quote is used within it; does that sentence taken on its own represent the greatest sentence ever written? Personally I don't think so. Are we to take sentences out of context and judge them in isolation from the work which contains them? If not then surely the sentences 'greatness' is at least in part due to the greatness of all the words within that work upon which it relies. EDIT:
Anyway, much to my own annoyance and against my personal sensibilities, the greatest sentence in terms of the impact it has had on Humanity has to be Genesis 1:1
In the beginning God created the heaven and the earth.