Canonically, Emerald is the strongest of the first 10, as in the epilogue of the emerald arc, we see that Professor Oak has Emerald's set of frontier emblems and he has all 7, meaning he won the tournament. However, we never actually got to see the tournament, so we don't know to what extent this is.
I say the strongest is Red. After all, we've seen him in action many times, and he's always come through. There are only three people who've ever beaten him (Bruno, Deoxys and Emerald), and he later beat the first two. Emerald...meh, the reason it wasn't shown is probably because there's no way that'd really happen. Red is the man.