Over the last couple of years I've put together what I consider to be a fairly impressive home bar, and I've spent much of that time experimenting with various cocktails.
I disagree. My guiding principle with regard to cocktails is that you should use good quality liquor that you'd have no problem drinking on its own. Calvados falls into this category, and so does Cognac. It doesn't have to be the really expensive stuff, but it should at least be middle shelf. The idea of making a cocktail out of foul tasting gut-rot in at attempt to mask the taste is anathema to me. When a cocktail is well made, the extra ingredients enhance rather than mask the taste of the main liquor.
My traditional ratio for a "sour" cocktail is 6/2/1. That's 6 parts base, 2 parts lemon or lime, and 1 part 2-1 sugar syrup. In concrete terms that translates into 6cl of liquor, 2cl of lemon/lime and 2tsp of sugar syrup. This yields a very sour, somewhat sweet drink, where the base can still shine through.
Lately, I've been trying out David Embury's classic ratio of 8/2/1. This is basically my ratio, with a third more liquor. Again, in concrete terms, this is 4cl of liquor, 1cl of lemon/lime, and 1tsp sugar syrup. I was skeptical, as this seemed like way too much liquor with respect to citrus. That being said, I gave it a shot (so to speak), and was surprised to discover that it blew my 6/2/1 ratio out of the water.
The liquor is quite clearly the main flavour in such a cocktail. In fact, I hesitate to call it a "sour" since it's not actually very sour. The citrus is definitely there, but it complements and supports the liquor rather than overpowering it.
This goes back to my main point, which is that you can't make this kind of sour cocktail with gut rot. The flavour is mostly carried by the liquor - the sugar and citrus enhance but don't mask it. So you need to use stuff that you'd drink on its own. And that means good whiskey or brandy.
So far, I've tried this ratio with rum, calvados and whiskey, and they're all pretty awesome.