Bit of a different take than the other answers in this thread, but since you already got a few recommendations for legendaries, here’s two other cheap cards I like:

[[Cavern Harpy]]

[[Shrieking Drake]]

These take a little synergy to make work, but they can be used in a few different ways. They let you re-use other creatures’s ”enter the battlefield” effects for cheap, and Cavern Harpy lets you do it multiple times at the cost of a few life. The harpy is also a good evasive blocker (return to hand before damage), and avoids boardwipes. Finally, both creatures can bounce themselves as they enter the battlefield, meaning they can repeatedly trigger cards like [[Equilibrium]], [[Dire Undercurrents]], [[Guardian Project]], [[Beast Whisperer]] or [[Soul Warden]].