If you mean "never":
* Nunca: This is the most common and direct opposite of "siempre."
* Jamás: This is a more formal and emphatic way of saying "never."
If you mean "sometimes":
* A veces: This means "sometimes" and implies that something happens occasionally, but not always.
* De vez en cuando: This also means "sometimes," but it emphasizes that something happens from time to time, not frequently.
If you mean "rarely":
* Rara vez: This means "rarely" and implies that something happens infrequently.
* Casi nunca: This means "almost never" and implies that something happens very rarely.
Examples:
* Siempre bebo café por la mañana. (I always drink coffee in the morning.)
* Nunca bebo café por la mañana. (I never drink coffee in the morning.)
* Siempre estoy cansado. (I'm always tired.)
* A veces estoy cansado. (I'm sometimes tired.)
* Siempre voy al cine. (I always go to the movies.)
* Rara vez voy al cine. (I rarely go to the movies.)
The best opposite of "siempre" will depend on the specific situation and what you want to convey.