What is the most heaviest mortar?

Answer 1

Schwerer Gustav - 1,350 tonnes
pls see, http://en.wikipedia.org/wiki/Schwerer_Gustav

2S4 Tyulpan - Soviet self-propelled mortar, weight 30 tons,
pls see, http://en.wikipedia.org/wiki/2S4_Tyulpan

Answer 2

The above answers while correct deal with mortars which are mounted on some form of tracked transport.
The heaviest man portable are normally the 120 mm Mortars used by Infantry Support Companies, they can be mounted on two wheels to help maneuver them across terrain but the wheels are removed before firing.