Love them both 100%. Both achieved different things, being different style movies:
GREMLiNS is often quite dark, though it has funny moments too. Both types of moments are strange in the best way. It tells a story with a lot of wonderful atmosphere, including the Jerry Goldsmith score for it. I love his entire score, and the songs on there that aren't by him are great too. Gizmo is adorable, and Stripe is a great villain. And along with all of this, the film has something to say; the man who originally owns Gizmo (not yet named Mr. Wing) is the man who says the key line regarding this.
GREMLiNS 2 is often goofing on the first film, (among other things) and it's a lot of fun! Director Joe Dante was told he could do whatever he wanted, so it's all over the place, yet still has social commentary (such as how technology isn't always great, for one thing). Though storytelling doesn't seem to be the main goal, the film doesn't suffer from that at all.
Both films were made at just the right time to capture the magic they did. If I had to choose, I'd go with the first, but that's not a knock at the second film at all, because it's just as fantastic in it's own way.