What was your biggest or most complex job?  For me it was adding a 40 gallon gas hot water heater to an existing 40 gallon electric electric.  I had a spare gas heater sitting around collecting dust.  The house was a 2BR with 3 real beds and 2 sofa beds.  I plumbed the gas supply line and the cold water inlet to the gas heater, routed hot water out of the gas heater into the electric heater, then installed the vent through the roof to vent the exhaust.  Gas heat is much cheaper than electric heat.  The gas heater heats up the cold water.  The electric heater keeps the hot water hot.

My 2nd biggest job was installing the hot/cold and sewer lines for 2 clothes washers that I put in a kitchen.