Usually use double fishermans. Used to use EDK. New book suggests Flemish / Figure 8 bend to join rappel ropes. What do you think? Lower profile and easier to untie. I like that.

use an overhand

+1 for overhand with long tails (easy to tie/untie and less likely to get stuck as you pull). But any of the options you listed will work just fine.

What book was it?

I dunno, seems like reinventing the wheel. A overhand is pretty much as simple as it gets. Easy to tie / untie, flat and simple. I haven't seen the flemmish, but the overhand is tough to beat.

Oh! Good!

This will be a worthwhile, congenial thread!

I use, and highly recommend, the EDK. But if you do choose to use the figure 8 bend, just make sure you use the in-line version. The off-set version has the same bad habit of an EDK but much worse, and no advantages over the EDK. (P.S. bad thing about EDK is that if not tied tightly, and backed up, it can roll. When it rolls, you get the same knot, but with less tail. If it rolls off the tails, you're done. Off-set fig-8 rolls easier, and eats more tail with each roll.)

If I'm not mistaken, a Flemish bend and a properly tied double (or re-traced) figure eight are one and the same. I personally prefer either the Flemish bend or just an overhand know with long tails for double rope rappells. I feel that the Flemish bend is more secure for joining ropes of different diameters, although I have heard that double fishermans are more secure in this circumstance, I feel that creates more of an issue with bending, snagging, or turning in the rope(s). Perhaps it's a matter of personal preference, and maybe an extremely experienced guide or well traveled Alpinist may provide a sounder response. In the meantime, though, I will personally trust the Flemish bend or an overhand with long tails. That's MHO though, please take it with a grain of salt.

Troll

Chris nails it.

Tripled bowlines with a trucker hitch finish is the safest. Of course this only works on ropes with thicker than 11.2. Everything else needs a quadruple bowline with a double fucker-nut hitch.

I prefer to splice the ropes ala Patrick O'brian

I use a Flemish.

It's best to just apply the belay knife.