Infernape is actually one of, if not, the best Scarf user to put on a Sun team. Think of the offensive things that typically give Sun teams troubles. Dragonite, Salamence, Terrakion, and Heatran are the big 4 imo. With a Scarf, Infernape wrecks Dnite and Mence, even when they're boosted, with HP Ice. You at least have a 50/50 chance to KO Terrakion so that Venusaur can possibly sweep unhindered. Hey, it's a better chance than what every other Fire type Scarfer will give you We know what Infernape can do to Heatran. CC makes Tran run for the hills. Sure, there's Darmanitan and Victini if you want more power, but Infernape brings what neither of those 2 can reliably bring. Utility in a Scarfer, neutrality to SR, and the ability to assuredly KO non-ES Dnite, Scarf Mence, and at least have a shot at taking out Terrakion. Honestly, if I have Venusaur or Volcarona on a Sun team, I definitely at least want to consider adding Scarf Infernape to get rid of the stuff that gives those 2 issues.
IMO, Scarf Infernape is the best one right now as it doesn't rely on breaking holes in teams (hard to do with rain being so common) but rather patches up common weaknesses that Sun teams have. Don't forget that Infernape has that same base 108 Speed Terrakion has to revenge Modest Venusaur in the Sun, only Infernape has Fire STAB to destroy it for sure with no prior damage. As long as Ninetales is higher up in the usage stats, Infernape will be close behind due to the utility the Scarf set brings to a Sun team.