Loudly Calling Out:
* Shout
* Yell
* Scream
* Call out
* Cry out
* Roar
* Bellow
Informal/Regional:
* Yell
* Whoop
* Screech
* Holler (this can be used as a synonym for itself)
Specific Situations:
* Call (for help)
* Whistle (for a taxi)
* Signal (to someone)
More formal:
* Exclaim
* Utter
* Announce
* Proclaim
The best synonym for "holler" will depend on the specific context of your sentence.
For example:
* "He hollered at the dog to get off the couch." You could replace "hollered" with "yelled" or "shouted" in this context.
* "He hollered at the top of his lungs." You could replace "hollered" with "screamed" or "bellowed" in this context.
It's important to consider the tone and intensity of the original sentence when choosing a synonym.