What is the most cost-effective SHTF ammo?

What is the best performing SHTF ammo?

Both questions have the same answer:

Whatever ammo you have loaded and ready to go WHEN the SHTF...!

Don't let the ammo-whores get to you  Buy what you can, rule of thumb has always been 1000 rounds for each gun you have.  This amount is at first a goal to achieve, then it becomes your low level limit.  Build up a little surplus so you can go shoot when the opportunity presents itself.  

My philosophy has always been to buy a little every payday ....  25K of 5.56, 308, 9mm, and 45acp.  since mid-2006 ... I usually look for American Eagle/Federal, Wolf Gold, WWB, XTAC, PPU, PMC Bronze in  55 or 62grn... (I used to shoot a lot of Wolf Poly and the stuff in the tan boxes but I am about out of it).  Lots of others available, these just seem to be what is in my area.  I also have some cans full of green tip...  So basically my stash is a large hodge podge of whatever was most cost effective when I bought it.  Nice to be able to just grab a can of ammunition and head to the range regardless of what the market is doing. I probably shoot 25% of what I buy/bought and store the rest.
I do have a couple of grab and go cans and for HD I have a couple hundred rounds of  62grn Fusion.

I work on a Naval base so if the SHTF during working hours, I probably wont even make it home...

Stock up, you can always take it to Barter Town...
Sorry to ramble.