Best where? There's no single answer. It depends heavily on which maps are in the rotation and which maps people like to currently play in custom games.
As for assaults:
On a large flat map in the relative early-midgame? Ravager.
On a flat map in most cases? Reaper.
On a flat map against other heavies or striders? Goliath.
In a situation where you need to first bypass heavy defenses to quickly raid vulnerable eco behind it? Halberd.
On a map with some water to escape or against units that are vulnerable to being skirmished? Grizzly.
Against stationary enemies (porc, blackholed or sumoheld opponents)? Jack.
