Tough to rank them when the belt has been in so many hands over such a long period of time.
Pedro Morales deserves some sort of mention, though I can count the number of Pedro matches I've seen on one hand. He had the belt forever between two reigns and helped to pioneer and solidify the belt.
In the 80s, Honky has already got a mention. Randy Savage definitely needs a shout out because when I picture the IC Title belt, I often picture Macho Man holding it. Given he was one of the top three, if not top two, guys in the company during his run he certainly brought up the prestige level. Plus the classic with Steamboat at WrestleMania III helped put a lasting spotlight on the belt. Then you must mention Warrior. Like Mach, he was one of the very top guys while he held the belt and he also took it into the main event of WrestleMania VI, which is probably the biggest stage it's ever been featured on.
90s had a lot of great champs between Razor, Shawn, Bret, The Rock and Triple H. Hard to say who stands out from that pack.
2000s also saw it held by basically all of the best workers of that decade. Jericho, Benoit, Guerrero, Edge, Christian, RVD and so on. I have to give a shout out to Randy Orton for his reign from 2003 to 2004. It seemed like he really came into his own while holding the title and it was one of the longer reigns of that era.
In the late 2000s and early 2010s I think it fell off considerably. The various runs of Kofi, Dolph, Drew, etc. were very unremarkable and at this point the belt really became an afterthought. The only person who really helped bring the title back time and again was The Miz. And I also gotta give props to Seth Rollins for his two runs in 2018, the matches with Miz and Ziggler were huge and the highlight of Raw for that stretch. He even brought the title back to a PPV main event with his Extreme Rules 2018 match against Ziggler.
Anyways. If I had to answer... I think I'd go with Macho Man first. Honorable mentions to HBK, The Miz, Jericho and Orton.