Jose Mourinho has coached at the pinnacle of European football, winning a total of 25 major trophies and outwitting some of the very best managers in the game.
But rather surprisingly, it was Martin O'Neill's Celtic side that gave the Special One the most stress during his managerial career.
Mourinho's Porto side beat the Hoops in extra time in the 2003 UEFA Cup final, with Brazilian striker Derlei scoring an 155th minute in Seville.
With Porto having gone 16 years without winning a European trophy, a huge weight off Mourinho's shoulders was lifted and he views it as the most intense game he's managed.
"That Final against Celtic was not the biggest win, it was not the greatest joy, but in terms of intensity it was my biggest ever game," Mourinho said, as per the Daily Record.
"The game ended and I thought 'the game is over and I am dead. It's still the most tense game in my career.
"I've played three European finals since, two in the Champions League. I've won a lot of titles, been involved in so many incredible games.
"But in terms of living with tension, intensity, with emotion raised to the limit, that game against Celtic beat them all."
Mourinho would go on to guide Porto to a Champions League win a year later, before taking the reins at Chelsea and winning back-to-back Premier League titles, as well as one FA Cup and two League Cups during his first stint.
He then dominated in Italy by winning a treble in his first season with Inter Milan, before winning La Liga and the Copa Del Rey at Real Madrid over in Spain.
The 56-year old returned to Chelsea and won a Premier League and League Cup double in his second season, following it up with Europa League and EFL Cups success during his first season at Manchester United.
He's had offers to return to the game since his sacking in December but told beIN Sports he turned three jobs down, saying "I didn't feel it was what I want."