Answer:
Draft horses. Draft horse breeds include Belgians, Shires, Clydesdales, and Percherons. Belgians are generally the biggest (as a rule) -- big ones are over seven feet in height, and weigh over 3,000 lbs! But there is considerable variation, and Shires and Percherons hold the records.
The tallest horse ever recorded was a Shire gelding named Samson. He was 21.2 1/2hh (7'2) and weighed 3360 pounds.
Brooklyn Supreme was also a Percheron. He stood at 19.2hh and weighed 3200 pounds.
The tallest horse alive (according to Guiness) is Goliath, a 19hh, 8 year old Percheron gelding