It's a bit like records, on any given day I'd give you three completely different answers. I couldn't pick 3 all time best, there's so many from around the world you could talk about, for some many reasons - but from my own collection, my favourites (tho oddly none of them rare) theatrical posters are -
Alien (2003 Director's Cut release UK Quad, silver mylar with glow in the dark egg cutout)
Superman (1978 US Insert - for me the best presentation of Peak's artwork, as it's full bleed too)
Kill Bill Vol 1 (2002 UK quad)
Non-theatrical fave - Killian's Who Framed Roger Rabbit (Style D) - The Jessica Rabbit "I'm Not Bad, I'm Just Drawn That Way" one. Gorgeous print on mylar.