I feel you. This is a big problem in the crossover genre I find!
This is probably my fave SPN crossover because Sam and Dean are perfect, and the story is focused on them, but the other characters are very good too:
Men of Legend (http://archiveofourown.org/works/748772?view_full_work=true) by hells_half_acre Merlin must summon heroes from another world to help save Camelot. Dean and Sam Winchester didn't even know they were summon-able. Gen, SPN/Merlin, H/C, case!fic.
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This is probably my fave SPN crossover because Sam and Dean are perfect, and the story is focused on them, but the other characters are very good too:
Men of Legend (http://archiveofourown.org/works/748772?view_full_work=true) by hells_half_acre
Merlin must summon heroes from another world to help save Camelot. Dean and Sam Winchester didn't even know they were summon-able. Gen, SPN/Merlin, H/C, case!fic.