There are many blunderes in this game, if I had to pick just 1 and call it the biggest blunder it would be 27) Rd1
"27. Bxb7+ 27...Ka7 28. Bxe4" If white did these 2 simple moves he would have been in an engame with 2 rooks and 2 bishops vs 1 rook and 1 bishop but he messed up bad and lost.