I've used only Noodler's Bulletproof Black for years (in a TWSBI Eco piston-fill) - it works great, and I realized if I was going to take the time to write extensive notes it was worth having some protection against water.

I previously used Waterman Black - it flows really smoothly, but it isn't waterproof. Also, it doesn't seem quite as dark as the Noodler's.