They’re all powerful in different ways. It’s hard to say who’s the “most powerful” (besides us, of course). Saint led a crusade against the Fallen to push them back from the City, and killed the Devils’ Kell with a -blam!-ing headbutt, and terrorized the Vex for centuries in the Infinite Forest until the Vex were forced out of desperation m to create a Mind with the sole purpose of draining his Light. Osiris is probably the wisest and most knowledgeable character, and unlike Saint he has survived the Infinite Forest and has even managed to control it to an extent. His understanding of the Vex network allows him to traverse time with ease, and no one else has displayed such a mastery of this. With this, he basically knows about everything before it happens. He was literally in several places at once during Six Fronts because he can use his echoes to fight. Felwinter had a reputation for killing Warlords with ease. He challenged Shaxx to 1 on 1 combat 3 or 4 times, and Shaxx killed him literally within seconds every time. Rezyl Azzir/Dredgen Yor. The first Guardian. Also killed a Kell, and his minions, by himself. Shin Malphur killed Yor, is known to have the fastest draw in the galaxy, and has now harnessed the power of both the Darkness and the Light. Then you have all the other Iron Lords, and plenty of other deceased characters that leave behind a legacy of incredible feats. If I had to pick one though, as hard as it is, I’d have a hard time not picking Osiris. His combination of power and knowledge puts him in a league by himself.