I was a big “The Doors” fan (had most of their LPs), so I might be somewhat biased, but I think Val’s best performance was in The Doors movie – some original band members even had troubles distinguishing his and Jim’s singing, although his ‘Jim voice’ would wear down very quickly.
Kilmer made his own audition tape, in which he made himself up to look like Morrison and performed various Doors songs, which impressed Stone and Doors producer Paul Rothchild enough to officially cast him. Kilmer then had to impress Morrison’s Doors bandmates, and when they listened to recordings of him singing Doors songs, they reportedly asked "Are we listening to Val or Jim?,” which more than proved that they had found the right man. His singing was so on point that they always used his voice for any scenes of live performance, which were huge endurance tests to film, what with all the lights and sweat and how quickly Val’s voice would wear down after a few takes.
https://collider.com/val-kilmer-the-doors-jim-morrison-best-music-biopic-performance/
Ranking of his movies: https://ew.com/best-val-kilmer-movies-ranked-11707566
Lastly, one paywalled article: https://www.rollingstone.com/tv-movies/tv-movie-features/val-kilmer-jim-morrison-the-doors-1235309396/