My Rotring 600 Lava is about the sturdiest pen I could find, but I don't think it has much potential as a kubotan. It is less likely to come apart in your hand, but it is not really big enough to develop good force. I think of it as purely a last-ditch option that has a little potential for poking.
It makes a pretty decent pen, though. Works at least as well as almost any other, takes a good and popular refill (Parker or Fisher). Sturdy. You could do a lot worse. I carry one in my shirt pocket, but rarely use it, preferring ergonomic desk pens.