Which fighters have signed for tougher fights than Joshua has at 21-0

[club fighter]

You can take Parker & Takam off your list as they both have displayed to be less than significant opponents.
Pulev, the jury is still out on him, and Whyte is getting a pass (for now) for just being plucky.

That said, AJ's resume isn't that intimidating anymore, is it.

[Clegg]

He has a good resume for 21 fights but it's hard to compare his career to others. He was a big name before turning pro and has been a massive draw from the start in a way few boxers ever have. So he can get anyone he wants into the ring whenever he wants, when someone wins a vacant belt on a knee injury he can outbid the other contenders to get a title shot. The last person I can think of with early career advantages like that was DLH, who won a belt at 11-0, moved up for a second at 13-0, at 21-0 he signed to move up and challenge 96-1 Chavez.

If you're giving credit for fights signed but didn't happen then Breazeale gets credit for being willing to sign for Wilder but Fury getting the chance instead. Plus he faced AJ. He also signed to fight Stiverne before he got KTFO by Wilder in the rematch. He signed to fight Charles Martin but that fight was cancelled because Wlad vacated and Martin took the IBF fight.

So yeah Breazeale has done well. Willing to fight AJ, Wilder, the guy AJ beat for the title, the guy Wilder beat for the title. Plus a dangerous undefeated opponent Ugonoh. And he's had one less fight than AJ.