Find the best ink you can, then find the best pen that will use that ink.

Ironically even when cost is no object, the best ink costs about $4 / 3 euros, a bottle and the best pen that will use it is the Staedtler MarsMatic for $28 / 20 euros

Ultradraw dries slightly slower than the universal version which means it is less likely to need shaking the pen to get it to flow again

In the course of fussing over the best fountain pens, I have learned that the quality of the pen flows from the quality of ink it uses.