A full teched Arbiter. I find him more frightening than a Scarab, tbh...
(Although nothing will match the satisfaction of killing two scarabs and a full tech Arbi in my most recent game within literally 30 seconds of encountering them... Hawks + Spartans + Cryo + Disruption = 15000 enemy resources decimated for a loss of 500 of my own (one hawk killed by myself as a spotter for the cryo...).
Never the less, a skilled player with a fully teched Arbi is scary as *** if they can hit-and-run properly....