Jackie Brown for me.
Of all his film's, I feel it has the most depth and best characterization. It doesn't get as much love and attention as most of the rest as it's more subtle in its presentation, but I really feel it's an overlooked gem.
Second place goes to Pulp Fiction, third to Reservoir Dogs, and 4th to Inglorious Basterds, all superb movies.
Jackie Brown is the only one I couldn't watch in a single sitting. I got so bored, I looked at how much there was left of it and when I saw I'd barely gone over the halfway point of the movie I went straight to bed and watched the rest of it the other day. That movie felt endless... to be fair, a lot of Tarantino's films feel longer than they are, but Jackie Brown was on a whole other level for me personally.
Reservoir Dogs was another one that felt like it took forever for me, despite being his shortest.
Then again, I did say I only like 3 of his movies, the other ones really did bore me a lot, with maybe the exception of Kill Bill Vol. 1. That one was almost enjoyable. Didn't like the second one at all though. For the record, the 3 movies I like are Pulp Fiction, Django Unchained and Inglourious Basterds, the latter one being the only Tarantino movie I watched multiple times.