I love flangers!! I'm not a fan of the overused airplane noisemakers, but rather the subtle movement they can provide in a way that is deeper and more complex than a chorus. They can do chorus, too, and get really close to a phaser or Leslie, too. Just a very versatile effect. I use them to add movement to lines or provide some diffusion when I want to blend into the background a bit while retaining definition. I think slower speeds are better unless trying to emulate a rotating speaker.
More or less in order...
Tychobrahe Pedal Flanger. Unique, so expressive with the treadle, just sublime.
Original MXR Flanger, although the reissues are pretty close. The original MXR Micro Flanger is great, too; the reissues of those are just not quite right. PastFX makes the excellent Hot for Flanger, which is based on the MXR.
ADA Flanger. Strange, weird, beautiful. PastFX's version, the 80/A, is a great version of this, too.
Ross Flanger. These are supposed to be heavily based on the MXR, but they have their own sound.
Hartman Flanger. Based on the Electric Mistress. PastFX makes the Electric Mattress and Retrosonic has a version, too, both of which are excellent. Nothing I've found from EH comes close to the original (which I don't have, yet). The old EH Deluxe Electric Mistress is really its own thing, and for me, not as good.
Boss Hi-Band Flanger. IMHO, just all-around better than the BF-2 which can get kinda muddy.
Boss BF-1. One of the originals and different than the later Boss flangers.
Ibanez Swell Flanger. Nice and organic sounding. Ibanez makes a few good flangers. The Digital Chorus/Flanger is one of my favorite digital pedal flangers, and the analog version, the Prime Dual Chorus, in flanger mode, is even better.
DoD 640. Like the Ross, not far from the MXR but still its own sound. Honorable mention to their later flangers, too, like the 575, FX75, etc.
Lots of others I've tried. Fulltone ChoralFlange and TC SCF are both really good. Strymon Deco is nice, too, not really a flanger, although it can do that. Every digital multi-effects had a flanger in it, a few were OK, but never quite measured up to the above for me. Eventide and Lexicon were the best in that realm, IMHO.
The one I haven't tried and would love to (just not sure I want to spend the money)... Mu-Tron Flanger... someday.