McClaren's men were 'out on their feet' in the dressing room after John Eustace's looping first-half header gave them a 1-0 win over their Sky Bet Championship promotion rivals at the iPro Stadium.
It was the first time this season that the Rams had beaten another team in the division's top seven and t he former England manager felt it was his best result since taking over the Rams reins at the end of September.
"It's probably the most satisfactory, yes, as it's against a top-three team," said McClaren, who spent three months as a coach at QPR prior to his appointment as Nigel Clough's successor at Derby.
"Results like this can only be good for belief.
"Generally when you win a big game the players are flat out on their feet, flat on the floor in the dressing room.
"And it was like that. All I got was smiles.
"When we first came in all we wanted to do was provide excitement for the crowd and get to the play-offs, and we've done that. But that is still our goal.
"We've gone from 14th place in the table to fourth.
"The top three have been there all season. Burnley have been up there all season and the target for Leicester and QPR was always to get promotion.
"We need to keep collecting points and see where we end up."
The game's decisive moment came after 20 minutes.
Rob Green came to punch away Will Hughes' inswinging corner but the ball merely brushed his fist and was diverted into the vicinity of Eustace, who looped his header back into the far top corner of the net.
"Gary O'Neil said he was fouled," said QPR boss Harry Redknapp.
"I haven't seen it again yet though, so I could be talking rubbish, I don't know. But the ref has given (the goal) now, hasn't he?
"We didn't create nearly enough chances, that was the problem.
"We didn't press them in the first half and we seemed more worried about them.
"We were better in the second half but we didn't created enough."
QPR lack firepower with leading scorer Charlie Austin and Matty Phillips both out injured for the remainder of the season.
Redknapp may pin his hopes on Manchester United youngster Will Keane to fire his side back to the Barclays Premier League.
The 21-year-old forward has yet to feature since arriving on loan at the end of January but Redknapp thinks he is an exciting prospect.
"Charlie was our goalscorer wasn't he, really?" he said.
"He's a big loss, and Matty Phillips is out for the rest of the season too.
"I may look at it (the loan market) but we've got Kevin Doyle, although he's been out of the goals at Wolves.
"But I'm looking forward to seeing young Will now, he's a good player and he will get a chance.
"It's an area we might be a bit short in but I still feel we have a big chance (of promotion)."
Asked about his interest in signing Ravel Morrison on loan from West Ham, Redknapp added: "I saw that story earlier.
"If they want to loan him he'd be of interest, that's for sure."
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