Chesnaught is the type of Pokemon you would use if you need a slow, yet hard hitting, defensive Pokemon. When used as a boosting sweeper, it is outclassed by Conkeldurr if it's slow, and Lucario-M if fast. As an All-Out Attacker, it's outclassed by Breloom. Generally, Chesnaught's great support movepool is too important to fit into a set, or outclassed. This is the best set we have right now:
Chesnaught @ Leftovers
Bulletproof
252 HP / 252 Def / 4 Atk (might change based on damage calcs for Excadrill and co)
Impish Nature
Spikes
Leech Seed / Synthesis
Low Sweep
Seed Bomb / Taunt
M'kay, fair enough. Low Sweep, though? It's not his hardest hitting move, and he's going to be outspeeding much anyway, right? Is there any point running Reflect on Chesnaught? He already has awesome Defence, so it's not much there, but...