Can you give more bands that you like to narrow that down? Metal's a very wide genre of music so just narrowing it down to clean vocals isn't super specific. I can give a couple suggestions for bands that I like though, be warned I like prog stuff.

Devin Townsend has been a favorite of mine for a long time. Amazing vocalist and musician in general, really creative and interesting but not for every one. Ocean Machine is his best album in my opinion but if you want something that's a little less serious check out like Ziltoid the Omniscient or Deconstruction. Also if you're into more extreme thrash stuff check out his older band Strapping Young Lad.

Mastodon is also a huge favorite of mine. Their older stuff was mainly growly vocals though but as they've gone on more band members started to sing and they all have different styles to accentuate what they're doing in a song. They do mixed vocals amazingly well because of it IMO. Crack the Skye is a top 5 album of all time for me so it's the one I recommend you check out the most but their other stuff is great too.