For ‘important’ stuff, I use a Pilot M-90, fine nib, with black Sailor Kiwa-Guro ink.
For every day notes, I use a Caran d’ Ache 849 ballpoint, or a Papermate Mirado Classic in HB (I have a ridiculous stockpile of US-made ones).
When I’m feeling fancy, I’ll use a Tombow Mono 100, or a Craft Design Technology Item 17 (both HB).