Varro Tigurius
Varro Tigurius is one of the most powerful loyal human psyker currently alive. And he belongs to the Ultramarines. Who would have thought? He is so powerful he can beat daemons like a Grey Knight, he can fight like a Terminator, use every known psychic power (and several unknown ones) AND foresee the Hive Mind's plans. Somehow. It is worthy to note this is the same Hive Mind that blots out a psychic power on par with the God-Emperor of Mankind Himself. How one man can use the warp against this is unknown.
In spite of all this, the Daemonculaba still gives him nightmares.
Also did we mention this was BEFORE Matt Ward wrote him? Except he was Ld9 because due to his awesome powers his soul just shine in the Warp like a giant neon banner with a sign "eat me daemons, I'm so delicious!" on it.
In game, think of him as a psyker that will mind rape everything. Then double whatever first impression you have and then you have a good idea of his prowess. Then again, a generic nameless Eldar Farseer or ML3 Chaos Sorcerer can unleash just as much (or arguably even more) mind rape as him, so take it with a grain of salt.