No Manchester United shirt carries more weight than this one. The 1998-99 Umbro home shirt was on the pitch for all three trophies of the Treble, and for the single most replayed two minutes in the club's history.
The Manchester United 1998-99 Treble shirt design
Umbro's template for 1998-99 was a red shirt with a white polo collar closed by a black zip, a black and white Umbro diamond pattern running across the shoulders, and matching striped cuffs. Sharp, United's shirt sponsor since 1982, sat across the chest in black.
After the season was won, Umbro produced a commemorative version with "Premiership Champions F.A. Cup Winners Champions League Winners 1998-99" embroidered around the club crest, and Premier League winners' patches added to the sleeves. That's the version most collectors mean when they talk about "the Treble shirt" specifically, as distinct from the standard-issue shirt worn for most of the season.

Camp Nou, 26 May 1999
United went into the Champions League final against Bayern Munich without Roy Keane and Paul Scholes, both suspended, and spent most of the match chasing the game after Mario Basler put Bayern ahead inside six minutes. As the ninety minutes ran out, United were still 1-0 down.
Then came stoppage time. Teddy Sheringham, on as a substitute, turned in a corner from David Beckham to make it 1-1 barely a minute into injury time. Two minutes later, from another Beckham corner, Ole Gunnar Solskjaer, also introduced from the bench, stuck out a boot and steered the ball into the roof of the net. United had gone from losing to winning in under three minutes, and completed the first Treble in English football history.
Why the Manchester United 1998-99 Treble shirt scores a 5
There isn't really a case for scoring this one lower. It's a genuinely well-designed late-90s Umbro shirt in its own right, but its status has nothing to do with restraint. It was worn for a title win, an FA Cup, and the most dramatic Champions League final of the era, by a squad that's still talked about as United's greatest. If a single shirt can represent a club's entire modern identity, this is United's.