I think it's a tie between the MPTL and AT-AT.
MPTL's are powerful with those torpedoes and there's no way to defeat them if they are defended by turrets and a repair station unless you use a bombing run.
AT-AT's are very good because of the endless reserve of stormtroopers it has in its' hull and can wipe out most vehicle support with the heavy laser cannons. Not to mention they have the most health of any ground unit.
"You will pay the price for your lack of vision!" - Palpatine