There are far too many factors to be taken into account to categorically state that one route is the best killer.
TDs have pretty awesome damage but they won't all fire because their initiative is so slow. In most cases you'll be better off with pure SAs rather than pure TDs.
Or on the AD front, extremists might have the highest AD(I'm taking Ant's word on this), but nothing armoured can stand up to RPGs because they fire earlier than any other unit, whereas extremists don't even fire range.
Personally I would say the most efficient killers are probably SAs and RPGs. They are both moderately priced, they both fire range, they have fast inits, they have decent ETAs, they are exceptional at killing pure armour or health...but that doesn't mean they are the best routes.