If you are talking lead singer for Van Halen alone, then it's Diamond Dave. If you are talking career, Sammy was with Montrose, had done well solo, and did well with Van Halen.
Roth was way more charismatic,flamboyant than Hagar for sure. And for me, Van Halen was a bit heavier too. I remember so much more in the 80's when MTV were playing videos. I remember Montrose but not familiar with their music. Looks like Hagar is touring again with his band and Van Halen touring too this month.