Have you tried an electrically conductive grease? I've solved flickering in a couple of maglites by scrubbing off all the old lube with alcohol (I was using petroleum jelly as the old lube), and applying deoxit gold as the new lube on the threads and rings.
Though if yours are completely dead, it doesn't sound like that would help. I've never had a maglite completely die on me, although flickering was a common problem when I was using petroleum jelly to lube them. Some worked fine with it, others were more finicky. On other brands, I haven't found petroleum jelly to cause any issues; but it seems that maglites are prone to poor conduction.
If you don't have any conductive lube, then perhaps scrubbing off any existing grease and oil, and not using anything, might be something to try.
I've heard that the twisty switches on maglites can be a point of failure. Maybe that's what happened to yours if they're completely dead. I don't think maglites are great quality, but they're usually good. 4 out of 6 being bad sounds like you've had either really bad luck or you're really hard on your lights.
Good lube is not a guaranteed fix. Some maglites just love to flicker, no matter what you do. Whack 'em and sometimes they'll stop for a few minutes. I've never had one die on me, but other than my old incandescent C's and D's maglites, I can't say I've been very happy with them. They're decent performers, but they don't make the LED lights like they used to make the old incandescents.