It depends on what you like. Do you want a super bright ceiling or something softer? Lots of people would suggest Decorator's White or White Dove, but I like Steam AF-15 better. Decorator's white is super bright with a blue undertone, White Dove is too soft and dark for a trim or ceiling color IMO. I have one ceiling in my house painted in White Dove, and stopped there. Then there are people who care less what type of white they paint their ceiling and will just use untinted ceiling white, straight from the can. I think it's worth the effort to try different whites directly on your ceiling to determine which you like best.

I liked Steam the best because it has a neutral beige undertone and is a crisp white without being too stark and will work with your wall paint. If you want something whiter, try Oxford White.

In my personal house, I prefer Falling Snow, by Behr.

If you like the wall color, you could have the paint store tint a can of ceiling paint to 50% of alpaca. For the most part it will be white, but will harmonize perfectly with the wall paint.

I very much prefer the softer transition of the 50% wall color. Actually, if you don't have crown molding and the wall color is neutral and light, which I imagine the color "Alpaca" may be based on it's name, I often prefer the ceiling painted the same as the walls.

Actually, now that I'm thinking about it, I was just in a greige room with 50% tinted ceiling where they weren't going to do crown and then added it later after the paint was on the ceiling. The crown is a bright white, and I still prefer the ceiling tinted, because if you are looking, your eye stops at the crown, you don't even notice the ceilings. It's very soft and subtle, and the ceiling doesn't appear to be a "color," even next to the white trim.

However, I do know a lot of people prefer bright white ceilings, so if you have a white trim color you are using with the walls, you might want to consider that for the ceiling in a more flat finish.