It's hard to say with Kofi because his singles success is very fresh, if not a tad spread out over the course of his career. Monetarily I wouldn't be surprised if he's made the most money of the three based solely on merchandising alone. Being the first African born world champion in the WWE certainly is a career credit that will go down in the history books, but whether or not it truly makes an impact depends solely on what happens after his reign. Will he get the title again? will he stay at the top of the card as a singles athlete? will his ascension open the doors for other black wrestlers in the main event? or has that glass ceiling only suffered a chip to it's exterior? These are questions that we don't know the answers to yet. As a faction/tag team he'll definitely have a legacy with New Day as being a long tenured trio that works well together and have had great matches with the likes of The Usos. If we're talking about Kofi alone, I'd say the book is still unwritten.
Miz similarly has a lot of caveats to his legacy. While true he held the WWE title, it wasn't a great run and was greatly overshadowed by the Rock/Cena feud, which even bled into Miz being the odd man out in his only Wrestlemania main event. As an IC champion he'll go down in the books with the likes of the Honky Tonk Man and Chris Jericho. As a character who creates moments, he's had quite a few career defining segments that will definitely go down in history, especially within an era that tries to fabricate moments that otherwise don't stand out as much as some of Miz's antics. Some of his biggest feuds have been mishandled I think. He and R-Truth against Cena could've been handled better for all men involved, the same goes for his couples feud with his wife against Cena and Bella that was solely carried by Miz and Maryse on TV, and the long built epic of Miz vs Bryan sort of petered out with a double turn and a separation before anything was truly settled. Even now Kevin Owens has usurped Miz's feud with Shane McMahon before it came off the blocks. Miz certainly has a record of career assists when it comes to getting feuds off of the ground, but I'd argue that he hasn't had that star making victory or a properly ended feud that made both parties leave with their integrity intact. I mean, the guy even played goon to his own stunt double at one point. It could be argued that Roddy Piper's win-loss record isn't anything worth writing home about and he's still considered a legend and an icon, but I don't know if Miz has had nearly the career high that Piper's had, and that's saying a lot considered Piper never won the gold. There's no arguing at this point as to whether there's something bankable or not about the Miz, it's just a matter of sussing out whether or not WWE has capitalized on it to it's fullest potential. I'd argue they haven't, but even with that said, in an era where the personality and individuality well runs pretty dry in the WWE, Miz is a font of self-made charisma.
And then there's Dolph....As a competitor Dolph holds no credibility. Even his biggest title reign was quite literally handed to him. Of course that doesn't have to matter when it comes to long term legacy, but I've always found Dolph to be little more than a good hand. It could be argued tha this career has been continuously mishandled, but even on the instances where it's been said Dolph was given carte blanche on creative direction he's fallen flat and failed to evolve. He's been shouting the same promo for ages now about being disrespected and undersold by the WWE and it's fan, and the words have started to ring hollow. I kind of chuckled when he was paired with Drew, because before he was signed Drew spent about 3 years in TNA and on the indies delivering the exact same promo everywhere he landed as well, but Drew grew. In an era where championship reigns mean nothing in length and everything in number, Dolph still feels like a paper tiger. He's had the same career trajectory to nowhere for almost a decade now. He's been handed opportunity after opportunity and fails to capitalize. I'm not one who believes in the fabled McMahon brass ring, but if it exists, Dolph's been chomping at it forever and has been incapable of pulling it down under his own weight.
I guess my answer as of now is Kofi in the number one spot, with potential to grow from this point onward or to fall off the map and into the dusty pages of a history book as a statistic. He's still made his money, he's still made his legacy with New Day, and as a singles and a trio member he's created plenty of great matches and memorable moments that people will look back on from years to come. Whether or not they look back with a sorrowful heart over what might've been or with a pride that this was the first of his kind all depends on what tomorrow brings.
Miz lands in second for me as well simply because I do think Miz will be remembered for his personality and some of his promo work. He has a few great matches under his belt as well, which may actually give him a leg up on Piper. That isn't to say that Roddy wasn't a great wrestler, it's just that when people look up Piper they're almost solely looking for the promos. Where he flounders that Piper succeeds is in being able to come back from big losses and carry out feuds to their final destination. There are a lot of 'what if's' with Miz's career that I'd love to see amended in the future.
Dolph's a failed experiment of pushing mediocrity. I wish I could say that's at least one unique thing about him, but he comes from an era where mediocrity was test driven and world titles were passed around to career midcarders in droves. When given a chance to do things on his own terms, he failed to do anything different and that's what sets the other two apart from a guy like Dolph.