Scotland's champions beat the Spaniards 2-1 – only the second time Barca have lost in any competition since April.
Celtic are now on the brink of qualification from Group G.
"This is up there with anything I have done in my life," Lennon said. "As a manager, I don't know if I will top this; I hope I do, because I am still young and am still learning. But this is up there with anything I have achieved, not just in my football career but in my life."
Meanwhile, Xavi says Barcelona must focus on their next Champions League game, a trip to Spartak Moscow in Russia on November 20.
"The important game now is when we go to Moscow," the 32-year-old told reporters.
"You have to be top of the group. We missed the opportunity to do that [against Celtic], but we depend on ourselves.
"We have to go and seek victory in the two remaining games."
Tito Vilanova's side remain top of Group G with nine points, despite their defeat in Scotland.