"I hardly/highly doubt it" [closed]
I hardly doubt it
means you doubt it only a little.
I highly doubt it
means you doubt it a lot.
Both are grammatical but mean quite opposite things.
I've never heard the term "I'd hardly doubt it"; you'd use that in case you were unsurprised were events to take a certain turn.
"Do you reckon she'll come back?"
"I'd hardly doubt it. It's not like she has a back-up plan."
You'd use "I highly doubt it" (note use of I/I'd) if you were dubious of the course of events.
"Are the cookies ready yet?"
"I highly doubt it, it only feels like five minutes."