What is the name of Power rangers episode that has all Red Rangers?

You're likely thinking of "The Red Ranger" from Power Rangers: In Space. It's a two-part episode, where all the Red Rangers from the previous series (Mighty Morphin, Zeo, and Turbo) are transported to the Andromeda Galaxy and team up with Andros, the Red Space Ranger, to fight Dark Specter's forces.

It's a highly acclaimed episode and a fan favorite!