The Eephus pitch is a very slow and high-arcing pitch that is thrown with an extremely low velocity compared to most other pitches. It is usually thrown as a surprise pitch, to throw off the hitter's timing and disrupt their swing. The name "Eephus" is said to have been coined by Pittsburgh Pirates pitcher Rip Sewell in the 1940s.
Here's a good video describing the Eephus Pitch
Here's some of the BEST Eephus Pitches
Here's some of the WORST Eephus Pitches
As for the top pitchers in MLB to use the Eephus pitch, there are a few notable ones:
Braden Looper: Looper is a former pitcher who played for several teams, including the New York Mets and St. Louis Cardinals. He was known for his slow Eephus pitch, which he used sparingly but effectively.
Trevor Bauer: Bauer, former pitcher for the Los Angeles Dodgers, has been known to throw an occasional Eephus pitch throughout his career. He is known for experimenting with different pitches and using them in unconventional ways.
Phil Niekro: Niekro was a knuckleball pitcher who also occasionally threw an Eephus pitch. He is known as one of the greatest pitchers in baseball history, and his unique pitching style helped him achieve great success throughout his career.
Barry Zito: Zito, who played for several teams including the Oakland Athletics and San Francisco Giants, was known for his ability to throw a variety of different pitches, including an occasional Eephus pitch.
R.A. Dickey: Dickey, a former pitcher who played for several teams including the New York Mets, was known for his knuckleball but also threw an Eephus pitch occasionally. He won the National League Cy Young Award in 2012.
It's worth noting that while the Eephus pitch can be an effective tool for some pitchers, it is not commonly used in modern-day baseball, as many hitters are now better equipped to handle it.