Mastodons, elephants, brontosaurs, moose, polar bears, 6-meter-tall giant sloth, and whales.

Any critter which subscribes to the "walking fortress" school of animal design. Nobody messes with megafauna, because they'll either step on you, jump up and down on you, swat you across the clearing, stampede over you or gore you. Even bull moose--which are pretty poor excuses for real megafauna--can overturn a VW Bug with their racks when sufficiently annoyed.

Intelligence is generally not necessary.

It's a nice strategy. You're spread out--and you have several inches of skin--so you don't have much to fear much from microbes. Your ratio of volume to surface area is large, so you can maintain your temperature easily.  Your natural predators, if you had any, would need to be even more powerful than you, which causes all sorts of biological difficulties. Your biggest worries are climate change and MassExtinction every few hundred million years.

It's a pretty cushy job. You're safe, at least until nature manages to evolve a small, fast-breeding predator which is insane enough to attack megafauna, and dangerous enough to succeed.  Or you were unlucky enough to be wiped out by a disease, ice age, comet, or other drastic climate change.

That's why haven't seen much megafauna recently. They're yummy, and surprisingly easy to chase off cliffs.
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Similar arguments apply to MegaFlora: trees and fungus.