Reasons why Kerchak from Tarzan 1999 is the ideal silverback
This is most seen in the opening with his ill-fated son, but realistically, all the juveniles in the troop would most likely be his offspring. Disney shied away from making that explicitly the case but you can apply this point to how he watches out for and protects (including risking his life for) the juveniles in the troop. Silverbacks who are good with juveniles tend to have the most successful and largest troops, which leads us to the next reason.
2. His troop is quite large (with many juveniles).
A large troop is a sign of a very successful silverback. It's a silverback's responsibility to keep his troop happy and mediate conflicts between troop members, so the larger the troop, the more work he inherently has to do to maintain it. A large troop means a silverback who's very good at maintaining relations with and between his troop members. And tying into the first point, the many juveniles are an extra point of success.
3. He's patient, accepting, and doesn't chase conflict.
This one might not be very apparent if you're coming from a purely human perspective, but it's true from a gorilla perspective. He acquiesces to Kala adopting a strange baby of an unknown species pretty quickly. Tarzan's survival into adulthood implies that even though Kerchak wasn't fond of him, he provided the same protection for him as he did for the other babies and juveniles. Despite his disagreements and tension with Tarzan, he permits Tarzan to stay in the troop because it makes his troop happy. He's also alright with Tantor the elephant traveling with them.
His choice of plan for the unknown threat of humans is to avoid them, protecting his troop. When he does encounter humans he uses physical displays to warn them off and only becomes violent when Clayton threatens the juveniles (and honestly that scene is a "justice for Kerchak" scene, it was dumb luck nobody else saw what Clayton was doing).
This point ties into the next point:
4. There are several older blackbacks in his troop who all follow him.
Several of the troop members seem to be older blackback males, close to becoming silverbacks themselves. I don't know if the gorillas in the movie are meant to be any specific subspecies but the point stands true for all of them (even the one subspecies that regularly has multiple silverbacks in a troop) that having so many males so close to becoming silverbacks themselves still living in the troop is a sign of a very socially savvy silverback. And this ties into the next point:
5. He's extremely well-respected by his troop, who are very loyal to him.
Other than Tarzan, Kerchak doesn't really have any conflicts with his troop. And while the troop love Tarzan and want him around, they often side more with Kerchak when the two butt heads. It's especially notable as these conflicts can be seen as Tarzan challenging Kerchak's role in the troop, and the others' reactions show loyalty to Kerchak as their leader. The older blackbacks in the troop also don't seem to challenge Kerchak. His role in the troop is very respected and his troop, including those who could theoretically challenge him themselves, choose to follow him even when his dominance is challenged.
6. He's very physically capable and fulfills his responsibility to protect the troop.
Running into a stampede of elephants to rescue a baby, fighting a leopard... Kerchak's very physically skilled and strong! Other than the tragic case of the wandered off baby at the beginning of the movie, Kerchak's successfully protected his troop from threats through all the years since then. Also, I know we need to have suspension of disbelief for the ages of the gorillas, but I do want to point out that realistically, Kerchak in the main part of the movie would likely be past his prime and possibly even elderly so it's even more impressive how physically capable he still is. I'll meet them halfway and still treat him as past his prime, though in Disney movie world he might not have been elderly.