Liverpool are leading 1-0 from the first leg of the last-16 clash but the English side were outplayed in Paris a week ago with keeper Alison's heroics keeping them in the tie.
Slot said his side will need to be special to dump one of the “richest clubs” out of the competition.
When asked if they need their best performance of the season, he said: "I do think so because they are the most complete team we have faced so far."
Slot’s side are running away with the Premier League title at home but he believes taking on Luis Enrique's PSG might be an even bigger task than facing English rivals Arsenal and Manchester City.
Liverpool drew at Arsenal earlier in the campaign but have beaten City at home and away this season.
The Reds boss added: "What I mean by complete is we have faced Arsenal and City. Not that there are big margins but the intensity they played at, combined with the quality because we are talking here about probably the richest clubs, maybe there is one club a bit further ahead.
"They have so much quality and a great manager because he has the team playing in a way that is not easy to play against.
"He brings the best out of every player and incredible workrate and we have to be at our best. But for example we didn't have ball possession at City away - the other richest club in the world - but at home we had a completely different performance against City than away."
PSG boss Luis Enrique b...