Israel would need a B52 or B2 to deliver the 30,000 lb bunker busters that the US has developed. It is believed that the biggest US bunker busters are needed to crack Iran's deepest nuclear weapon research and uranium centrifuge containing bunkers.
Israel has requested 50 BLU-113 warheads. The BLU-113 is a 5,000 lb penetrator used with the GBU-28 laser-guided bomb that is designed to destroy deeply buried and heavily fortified targets.
Israel has a few thousand 2000 lb bunker busters
The BLU-113 is a penetrating munition that uses a modified Army artillery tube as the bomb body. They are fitted with GBU-27 LGB kits, 14.5 inches in diameter and almost 19 feet long. The BLU-113 is a 4,444 pound penetrator. The case is made from HP9420 alloy steel to provide target penetration capabilities. The bomb body is loaded with 670 lbs of 80/20 tritonal explosives and utilizes a single fuze.
SOURCES- Janes, Global Security