I didn't realize how many outfits can be unlocked via certain escalations/challenges until recently, so I've been mopping those up.
Arkian Tuxedo - Complete Blake's Endeavor challenges in Sgail
Buccaneer - The Bartholomew Hornswoggle escalation on Haven
Cashmerian - Dubious Cohabilation escalation on Mumbai
Blue Flamingo - Spring Breaker Challenges on Miami
Imperial Classic with Gloves - Complete Divine Descendance escalation on Mumbai
Santa 47 - Holiday Hoards on Paris
Snow Festival - Blood in the Snow on Hokkaido
Summer Suave Suit - The Calvino Cacophony escalation on Santa Fortuna
Tactical Wetsuit - Play The Mills Reverie escalation on Hawke's Bay
For items, I'd say the standout ones are things like...
Sieger 300 Ghost - Best sniper in the game, high damage, silenced and 4 levels of scope. Can get if from The Cheveyo Calibration on the night time Marrakesh map.
Sieger 300 Advanced - I think this is mastery 20 on Hokkaido in hitman 2? It plays Ave Maria when you scope.
Krugermeier 2-2 - Bangkok mastery 20, quietest silenced pistol in the game.
El Matador - Delgado Larceny escalation on Santa Fortuna. Loud pistol that is hilariously dumb to use. Pierces bodies, makes bodies go flying back when you shoot them.
Striker - Same as Matador, Dexter Discordance in Hokkaido.
Custom 5mm - Hokkaido mastery 5. Pistol that isn't detected on frisks.
Concealable Knift - Hokkaido mastery 20. Knife that doesn't get detected in frisks.
Sieker 1 - Haven mastery 10. Pistol that shoots emetic poison rounds, suuuuper strong.
Kalmer 1 - Whittleton mission stories unlocks, tranq pistol.