Simple dry Irish stout:
90% really good ale malt (e.g. Floor malted Thomas Fawcett's Maris Otter)
5% Roast barley
5% Flaked barley
Fuggles (or any hop that provides smooth bitterness and is from the UK) to 40 IBU
A neutral ale yeast. Wyeast Irish Ale or similar. Nottingham goes well for dry. SO4 should work well.
That's it!
So, why is this good? It's pretty much Guinness, more or less. I personally think stouts really benefit from a solid, flavourful base malt, so that's why that's in there. Still works with any old base malt of course, just not as tasty IMHO.
Now Guinness do one extra thing: they add a bit of lactic soured, double strength stout back into the brew to give it a tang - different portions in different markets (or at least they used to - its all a bit dumbed down now.)
I achieve this by going and buying half a dozen Guinness stubbies (NOT cans - the 6% ones). The day before I open 2 stubbies, pour into a clean ice cream container and throw in a handful of crushed ale malt. The little lactose guys that live on the grain husk go to work. On brew day, strain the solids out and add the soured liquid to the boil for the last 15 minutes.
Or you can just add some lactic acid to taste once its completed. That's probably safer, but I'm a mash basher. I get a bit of variation using my approach, but so does fine wine and I'm okay with that. The beer is always great, but sometimes its sensational. YMMV
Think I'll go and have a stout now. That's what the other four stubbies are for.