Do you want to know what best-kept secret Ananda had, and why Sakyamuni Buddha said "none of the attendants of past and future Buddhas can outperform Ananda"? Ananda was the best attendant with multiple incredible qualities. He vowed to deliver sentient beings, serve Sakyamuni Buddha and help Buddha to deliver sentient beings. He was not a bad practitioner despite that Buddha scolded him a lot or he breached the commandments many times, or even he didn't practice well. He practiced spiritually life after life, was bodhisattva life after life and supported Buddha life after life. He would support any Buddha that came out.
Sakyamuni Buddha told Ananda to keep Buddha’s teachings in the dark age of the Dharma and to teach sentient beings so that they wouldn’t be cheated by Maya or tricked out of the right way. Buddha also asked Ananda not to go to Nirvana. So when Ananda went to Nirvana, he used his physical body to go while his spirit stayed. He might be reborn in many places or share his wisdom with many Masters in order to support and protect them, so they can teach sentient beings the right way; this means Ananda is formless and he can share his light and power with a truly good and pure Master. Therefore, anyone who teaches the Dharma with true teachings is Ananda himself.