Paul Nicholls' imperious nine-year-old stretched his long unbeaten run to 16 and continued his domination of the staying hurdle division which began in 2009.
But Ruby Walsh's mount was given a fright before landing the four-timer in this Grade 1 contest by 20/1 shot Voler La Vedette.
Big Buck's hit the front with two left in the three-mile feature and was being tracked by Nicky Henderson's classy Oscar Whisky but Voler La Vedette charged up the hill, under Andrew Lynch, and was just a length and three quarters back in second.
French Champion Hurdle winner Oscar Whisky failed to stay with Smad Place (20/1) completing the places.
Sir Des Champs took the opening race of St Patrick's Thursday for Ireland, winning the Jewson Novices' Chase.
Willie Mullins' charge, who landed the Martin Pipe Conditional Jockeys' Hurdle at last year's Festival and who has done well since switching to fences by winning Grade Two contests at Limerick and Leopardstown in his last two starts, aaccelerated up the hill away to win by four and a half lengths from Champion Court.
Sir Des Champs, owned by Ryanair chief Michael O'Leary and ridden by Davy Russell, went off as the 3/1 second favourite behind Peddlers Cross who never featured.
It was a third win of the meeting for trainer Mullins with an Irish 1-2-3 completed by For Non Stop (8/1) who was a distant nine lengths back in third.
The six-year-old received a 12/1 quote for next year's Gold Cup after the victory and Mullins said: "He was very good and I'm really looking forward to next year over longer trips.
"He was foot-perfect. Davy asked him plenty of questions and he came up every time. Davy rode him with great confidence. A good pace and plenty of jumping will suit him best."
Cape Tribulation claimed the Pertemps Final, the 14/1 shot winning the ultra-competitive handicap.
It was a patient ride from Denis O'Regan whose mount charged through on the long run to the final flight and won by three quarters of a length from Catch Me (14/1).
The 33/1 chances Cantlow and Houblon Des Obeaux completed the places.
Veteran Buena Vista, running in his eighth festival, failed to complete a hat-trick of wins as he faded after making the early running.
Malcolm Jefferson's eight-year-old was the star and winning owner David Abell said: "It's amazing for a small trainer to have a winner here, usually the big trainers clean up.
"He won a Grade Two by 20 lengths but it's been downhill ever since. We put him back over hurdles and it seems to have worked. Malcolm always said he was really good."
Riverside Theatre got up at the death to take a thrilling Ryanair Chase, the first of the day's two Grade One races.
The Nicky Henderson-Barry Geraghty success story at the Festival continued as he beat out Albertas Run (10/1) by half a length.
The double champion had a great jump over the last to lead while Medermit (8/1) was also in contention with the French grey eventually third, just a further half a length back.
Henderson, who became the most successful trainer in the history of the Cheltenham Festival on Wednesday when Simonsig won the second race, saddled his sixth winner of the week while Geraghty is now the clear favourite to be the top jockey after his fifth victory.
Actor James Nesbitt, who part-owns the horse said: "I just can't believe the ride that Barry gave him and Nicky has done such an incredible job to bring him back like he has.
"The training performance at Ascot was extraordinary, but to have him in that nick today - to have that courage and battle like he did is incredible."