I think that almost everyone will say a double butted spoke, but some will say the ones with three different diameters (at the nipple, midsection, at the head) are better.

I mentioned above that I am using 2.0/1.7/2.0mm on most of my wheels, that is Wheelsmith DB-14. If I have a choice, that is what I buy. But as I mentioned above on one wheel I used straight gauge because I was unable to find the length I needed in the DB-14 spoke which is what I wanted.

My last set of touring bike wheels I used Sapim nipples on the Wheelsmith spokes. I plan on using them for all future wheels.

The DT Swiss that are also popular for touring I think are 2.0/1.8/2.0mm. I am not sure but I think that some European companies use Sapim 2.0/1.8/2.0mm spokes on touring bikes.

That said, I have not broken a spoke for many years and the last time I recall actually breaking a spoke it was on a bike that was from the 1960s with a spoke that was that old. In the past decade and a half I have built up four wheel sets, three of those wheel sets I have toured on, have not broken a spoke on any of them.

There are thinner spokes that have a middle section of 1.5mm, but I would not use them on a touring bike.

Given a choice, I always go with 36 spokes per wheel.

Although I am not breaking spokes, i am still carrying spares on tours.