Reservoir Dogs. It's got an actual plot to it, it's got more Steve Buscemi, it has one of the best opening moments in movie history (not the walk, the diner talk), it's probably Tarantino's most restrained directing credits and it's what kicked off his universe of films. It also feels more old school cinematic to me. Like 12 Angry Men, most of the movie takes place in one particular setting and relies on the actors to work with their talents rather than an overly stylized film. That's probably because it's Tarantino's first film and it was shoe string, but it works in it's favor. Pulp Fiction is beautiful, and it's a cool movie and a wonderful kick start to Tarantino's true style, but the acting wasn't great. As a matter of fact, I'm sure it was meant to be wooden in some parts, or corny, or exploitation hammy. That's the vibe the movie was going for, it's kitsch at it's finest, but Reservoir Dogs was a straight up heist flick with the most no-nonsense, anti-heroic, sympathy for the devil humanity behind it. Sure, the mobsters were sociopath dicks willing to kill one another over a color name, but they also had their vulnerabilities and humanity. Except for Blonde. Blonde's nuts.
Also, above all else, at the risk of sound hipstery for a moment, I think Reservoir Dogs is more fringe. It's too violent and vulgar to edit down for TV, so the likelihood of seeing it without consciously going out of your way to see it (or having El Rey/Sundance/IFC, which I can't afford) is pretty slim.
It boils down to the fact that I watch Pulp Fiction because it's on and I watch Reservoir Dogs because I want to.