The first flanger I have ever heard and tried in my life was the EHX Electric Mistress in the 1970's. That's what 'flange' was supposed to sound like, at the time; it being modeled on what happens when you vary the speed of a tape over the playback heads and mix it with the original signal. Personally, it was an interesting sound for about 5-10 minutes and I didn't find it useful after that.
The best flanger I have ever heard, used and owned is my ADA S1000 rack delay (hey, while posted in pedal room, no one specified it had to be a pedal; just said best flange...). The ADA S1000 rack delay has the widest, deepest jet engine sweep of anything I've ever heard, it even bested the ADA Digitizer 4 I also owned. In fact, I've kept the ADA S1000 all these years just for that one reason - the depth of the flange. I've tried Roland units, BOSS pedals, and others and many sound pretty good these days, but nothing came close to the effect I got out of the rack unit.