Depends on how you define strength. Being the strongest Doujutsu doesn't mean automatically that it is best for the fight. The Rinnegan is the reincarnation eye which allows you to stay between life and death, this is the Rinnegans true power. The Sharingan on the other hand is suited for fight only. Related for fights, the Sharingan is most likely the perfect weapon. It is more varied, more precise and more devastating than the Rinnegan. Unlike the Rinnegan, which is specialized on Ninjutsu, the Sharingan allows you to use not only powerful Ninjutsu (Amaterasu, Susanoo and the techniques you can copy) but also powerful Genjutsu and excellent Taijutsu (due to the copy ability, too). The copy ability gives you a much faster learning progress and the see through ability a huge advantage in a fight, too. Even a Sannin failed in front of a mere Genjutsu, used by a 13 year old Sharingan user. The techniques of the Mangekyou Sharingan consist of fast and precise on shot techniques. For Tsukuyomi, a mere eye contact is enough to defeat all opponents except Uchiha with Sharingan; for Amaterasu, a single glance is enough to hit the target with an inextinguishable flame which burns, until the target is burnt to ashes. The third technique, Susanoo, is a gigant, ghostly samurai which is ultimately strong, fast and protects its user from any attack. Beyond the Eternal Mangekyou Sharingan allows you to use the perfect Susanoo, which is huger than Madaras meteors and can crush a meteor and two distant mountains with a mere swing of its sword - not even the Biju have a power comparable to this.
Kotoamatsukami, Izanami and Izanagi are also techniques which can be awaken by the Sharingan if you are a very capable Sharingan user.