"Is Zoro more powerful than Luffy"

- With a sword, I'd say they're even. Without, Luffy is stronger.

"Cooler than Sanji"

- Hell no.

"When will he be the best swordsman"

- When the series ends.

About Zoro in general... at first I just saw him as a generic swordsman, but he's grown on me, and the series does need it's obligatory swordsman who is just overall strong, badass & likes to sleep.