Scorchers seem the best raider unit for early com kill, due to speed and insane dmg once closed in.
And then ofc Ravager is the king of any non-rampart/somewhat open FFA.
Ravager/stumpy in essence is the TANK of the game. By that I dont mean it is the strongest, I mean that it is JUST a tank. Not a riot/rocket tank like ogre, not a heavy or superheavy tank like mino or goli, not a lightning or fire or light or amphibious tank etc. it is just a tank.
As such almost all mobile (they are very good against static except for deso)units can trade costefficently with small, unaided groups of them. To make them usefull you either need support (add riots or scorchers for dmg, though depending on what you choose that might cost you on speed), OR you use THEM as support (can work pretty well on large open maps in the pot, essentially your raveger group roams the front line putting a stop on any enemy breakthrough and/or adding strength to your allied assaults), OR you mass them to such a degree that they become overwhelming, at which point you can destroy (almost) any target at will provided you are willing to sacricfice.
It is the last one that makes them so efficent in FFA. In FFA cost efficency is much less important then the ability to quickly shift large amounts of firepower/army value to other sides of the map, and no other unit comes close to Ravagers ability to do that. Any other raider(and yes, once you get to 20+ of them they count as raiders far more then as assaults) is suceptible to splash dmg, with R. you can just drive in, regardless of defences. Again, its strength comes from the ability to be where the enemy troops are not (or are outnumbered) and once you achieve about 2 to 1 in army size(by metal not by units) for a particular skirmish even cost efficency wont even be a problem.
These kind of rushes CAN work in the pot. Once you get to about 30-40 R. you can storm almost any position. The problem here is the follow up. This can work really well if allied troops are pushing just behind, then you break the enemy position which can give you an in into their heartland plus your guys can reclaim. On the other hand this can go horribly wrong if there is no follow up, because then you just made a big metal donation to the enemy.
Its other strength(particularly in FFA) is that just like it is not a VERY GOOD unit against anything, it is also not a VERY BAD unit against (almost) anything. WHich is important in a FFA, where I might have to build units (or just dump metal into something) without knowing what units or even which enemy they might end up being used against.
Just my 2 cents :-)