Hard to get into, as in do, and be sucessful is different from hard to get selected.
I think this largely depends on how competitive the field is for specific designators at any given time. Right now, the tighest ones seem to be (in no particular order)
SWO
INTEL
PILOT/NFO
SUBWAR designator is extremely hard too, because of the schooling and qualifications required, but i don't know how competitive it is.
I'm still in the waiting phase, I'm applying BDCP for INTEL/SWO/CRYPTO, but from what I gather, ALL Officer designators are competive right now. Many people are appecting pro rec's to their third choices