Worst movie

I couldn’t finish it, but I’ve never liked any of his stuff. This was so awful, I thought Wes Anderson had made it.

Either Brothers Grimm or Brazil

I haven't seen all of Gilliam's films (Tideland and The Man Who Killed Don Quixote have not graced my eyeballs yet), so excluding those two, I'd go with The Brothers Grimm. I don't mind TBG, actually, although I know a lot of people do. The Brothers Grimm's CGI just isn't as endearing as puppet monsters (like in Jabberwocky) and it sorta hampers it for me. It also just feels a little "conflicted" and the tone just isn't as juicy as in other Gilliam productions.

For posterity, here's my ranking:

Brazil - sublime in every way from aesthetics to performances to exploration of dystopian/bureaucratic BS themes.
The Fisher King - truly touching and, while imaginative, one of Gilliam's more "grounded" movies. Such great work from Robin Williams and Mercedes Ruehl.
12 Monkeys - great sci-fi with top-grade performances
The Adventures of Baron Munchausen - I love this movie's spirit, heart, and championing of the Imagination!
Fear and Loathing in Las Vegas - a great adaptation of Thompson's strange work; gonzo, indeed
The Imaginarium of Dr. Parnassus - a lot like Munchausen, and while it isn't quite as good as that film, I love the bizarre world presented and the moral ambiguity at play. Plus, Tom Waits.
Jabberwocky - a bit dodgy in terms of rambling structure, but it's funny and the aesthetics are superb.
Time Bandits - I know this is supposed to be one of the best, but it's just kinda "there" for me. Great vibe, really fun, but wasn't as memorable as other works by Terry G.
Zero Theorum - Brazil lite, it feels like a re-tread, even though it's not. ZT is a bit of a hot mess for me. Still very thought-provoking, but it feels a little more patchwork than Brazil.
The Brothers Grimm - yes, the worst one, but I still find the two leads fun and the vibe of the film is neat. It's neither as funny nor as spooky as it could have been, though, and as I said, the CGI doesn't help. Still, I do kinda like it as a (very) flawed gem.

I didn't include Monty Python and the Holy Grail. I felt like that wasn't really a "full" Gilliam movie. If I included it, it would rank highly - somewhere around 12 Monkeys - but a lot of that is because of Python as a team, not Gilliam in-particular, so I left it off.

I haven't seen all his films, but out of the ones I have, I'd say Fear & Loathing in Las Vegas (1998) Given the source material, he could've done so much better.