I also favor single flute & zero flute countersinks. They work the best for me.
I bought a box of countersinks & centerdrills from a local retired machinist. Most of the CSs are single flute, all US made. In the box were also some multiflute CSs, some with spiral flutes. I mainly use the single flutes on my lathe. The multiflutes also work well but I find I need to feed them much faster/harder than the single flutes to avoid chatter. Like the others I spin all my CSs slow.
In the box were also some of those threaded shank CSs with pilots that screw onto an extended shank. Not sure what they are called but they are used in the aircraft industry. The guy used to work at Lockheed or something like that. They're 2 or 3 flute & also work well but I only have them in smaller sizes. Great for reaching into a hole or a hole that is in a deep area.
On my mill I like using the zero flutes. On my mill, sometimes the single flutes won't countersink concentrically with the hole, they'll sometimes wander. I thought maybe the CS I used was dull or something but it happens randomly. I'm sure it's just my mill's quill. Multiflutes don't wander for me & neither do the zero flutes. Doesn't happen on my lathe though with single flutes and the multiflutes tend to chatter on my lathe.
I got a set of KEO cobalt zero flute CSs brand new in case for dirt cheap, I love them. From the same seller, later I also got a set of KEO 6 flutes. I haven't used them much but they were dirt cheap too so I hopped on the set.
Why is Weldon CS so popular? Is it the range that they can CS? I have not seen another CS brand that has this range.
When I was searching for zero flutes, it seems like the Weldons are most popular even though others make them now. They don't actually have Weldon shanks, it's the brand as you know. Maybe back then they were the first to make them?