Unironically, my favourite is the standard Bic Cristal medium 1.0mm in black, but placed inside a Bic Round Stic body that I’ve reused for many years (I prefer a plain gripless thin body, since I use a gentle finger-leverage posture for applying pressure variations which I learned from the Palmer script method)

Or, for gel-based, the Uniball Signo black ink is my second choice. I particularly like using the Signo GelStick 0.7mm which I can make line variations from hairlines to super broad strokes (again, the leverage-based force application is key for effortless pressure variation techniques)