Which attributes/abilities to determine who's best at SLWR and SLCB?

It depends on your playstyle for the most part.

I run a lot of short routes from the slot, like some whips/slants/drags, and also a lot of corner routes. So I look for SRR, MRR, speed, agility, COD, CIT, etc. 
For abilities, I look for things like slot-o-matic, short elite/SIE, SOE, grab n go, human joystick, mid out elite/mid elite, RAC catch abilities, etc.
I do not put an emphasis on height for my slot WR, but it never hurts to have height on your WRs, no matter where they are positioned.