I feel like this is a hard question to answer because exactly what is "basic" will depend on the style... But as far as fighters who have simply mastered the absolute fundamentals of their given style, and don't use too many "tricks" or fancy elements, Bivol comes to mind for me. He obviously has a lot up his sleeve, but I feel like his main strength is his relentless application of near-perfect defensively-responsible pressure. He looks almost the exact same fight to fight, and has become a notable champion that way.
Probably Zhang. He’s old, he’s fat, he gasses and isn’t particularly fast. But has good timing and is fundamentally well schooled. So he can still whoop up on the youngsters who haven’t been schooled well.
Riddick Bowe mastered the basics IMO. Hard to pick out a weakness from him bar perhaps defence at times. Larry Holmes also the guy for the one-two combo IMO and could get in a war at times. Currently - I'd back Bivol probably.
Jermall Charlo doesn't have many frills to his style but there isn't anything he can't do very well. An incredibly fundamentally well-schooled fighter.
Currently Bivol. Usyk can do a few fancy things like double up with one hand when combination punching. He sometimes also jabs while keeping his head inside the centre line instead of outside the centre line to confuse opponents. Bivol is extremely textbook.