I don't think there is such a thing as "one bag." You might as well figure on having an assortment, and a cart or wagon to tote them around!
My primary 'bag' is a little pouch that fits in may back pocket. I might carry a ticker, flashlight, knife, strippers, 10-N-1, plus one other tool in it.
This is my main bag:
http://www.goclc.com/ProductDetail.aspx?sku=1509 . It's basically a traditional electrician's pouch with a stout handle and a zippered flap to keep most things from falling out.
If I'm going on a roof, chances are I'll be using a few tools that you don't usually associate with electrical work- things like wrenches and sockets. There's also the matter of either carrying the bag up there, or pulling it up on a rope. For rooftop jobs, I have one of these set up:
http://www.toolpak.com/html/PACKStoolpak4panel.html . This is the smaller of the two offered; the large one is the original tool backpack, the one that got it all started. Amazing, isn't it, how everyone else is offering tool backpacks now that the patent is expired! Anyway, this one differes from most in that there is little "free space,' instead holding every tool in place- no pile of stuff in the bottom of the bag. The front pockets are plenty large for a meter and (most) cordless drills.
Otherwise ... look at your work. I mean, just how often do you use a fish tape, a bender, and and a Romex stripper at the same time? There's a lot to be said for having separate bags for pipe work, wire pulling, finish work, and troubleshooting.
Which, naturally, brings me to my #1 kvetch about 99% of "electricians' " bags out there: there's no place for the meter! Or, for that matter, a cordless drill. Our bags are stuck in the 1950's. I'll give Klein credit for discovering that electricians sometimes carry fish tapes (nice touch on their backpack).
Buckets have their place. I prefer the rectangular ones kitty litter comes in- plenty tall enough for a caulk gun. Great for holding parts - be they tiny screws or sharp-edged boxes. Handy for hauling up a roof with a rope, too.
Finally, I'll give Werner a gold medal for these trays that attach to their ladder tops:
http://www.lowes.com/pd_304360-287-AC50-JB-3_0__?productId=3031875 . Besides ladder use, it's handy for keeping your stuff straight as you belly-crawl through crawl spaces.
