As a dedicated Lemon bait I reckon Cray is overrated, It has its nights but I have frequently out fished cray fishermen by using paddle crabs (when targeting Lemons). Just a few weeks back my mate threw out a pile of cray tails and started stealing my crabs because he got bored of not catching anything. Within minutes he was onto a big Lemon.
IMO by far the most effective Lemon bait is Crab, then Crayfish, then either cooked or raw prawn. Then probably a bunch of other Crustaceans which I have never used for bait. Then Squid, then baits like pillys.
But Cray has to be one of the best generalist baits, most desirable species do take it.
I honestly can not choice one ultimate bait, but would probably be either Squid or Cray. When targeting a specific species I rarely fish either.