Stargate for sure. I've only ever played X-Files on Visual Pinball, and while I do enjoy it (Love the music and the challenging playfield design), the rules really are incredibly shallow and I get bored of it pretty quickly. Stargate on the other hand offers lots of Strategy and overall has more going on, and I think the fact that it's been my only pin for almost two years now and I'm still not bored of it says something.
One warning about Stargate is that it barely has any flow (It can get pretty fast though, with all the kicker targets and guards that can block off the VUK shots). Even the five-way combo involves shooting three VUKs. If you're looking for a game with good flow, I might go for X-Files instead.