Depends what your tastes are. Most if not all of the early Spanish SWs have an American flavour, so a liking for traditional American Westerns helps.
Joaquin Romero Marchent is by far the best Spanish exponent of Westerns. While The Implacable Three only works in certain scenes, Gunfight at High Noon (with Richard Harrison and Claudio Undari) is excellent, as is Seven from Texas, which features more character development than usual and an emotionally involving story. Great scenery and photography too.
Most other examples I've seen, for example Ride and Kill and Relevo para un pistolero, are rigidly Americanised, lacking Romero Marchent's vigorous direction.
And the early US-Spanish Westerns, such as Gunfighters of Casa Grande and Son of a Gunfighter, are similarly lacking in excitement and innovation.