What's the best fast move for breloom with grass knot?

Bullet Seed is better most of the time if you're using it as a grass attacker. Counter is better against rock types that aren't also water or ground.

Bullet seed, the cases where fighting and grass hit for SE damage is limited to I believe bebarel and water ice types, and rock types. However most rock types are part ground, thus making bullet seed the better fast move.

So essentially counter/GK for Regirock, water/ice types like lapras, bebarel. And bullet seed for basically everything else.

However, Counter/GK/DynPunch is a good set for gym clearing, highest DPS fighting set, and after molitic day, there may be a lot more of them around behind blissey, so grassknot can easily deal with them

if you're using it for PvP, Counter is way better. Even if your foe resists Fighting, or is neutral to Fighting and weak to Grass, Counter does more damage and generates a hair less energy.

That's just a function of how good Counter is in PvP. Even without STAB, counter is the third best fast move in PvP, behind Counter with STAB and Dragon Breath with STAB. (Ranked by product of Damage/Turn and Energy/Turn.)