This is what I’ve been using: https://www.jetpens.com/Uni-ball-Jetstream-RT-Ballpoint-Pen-...

Although lately I’ve been using this multipen: https://www.jetpens.com/Uni-Jetstream-3-Color-Ballpoint-Mult...

It’s nice having 3 colors of ink available, and it’s not that chunky blue thing we had in elementary school.