what ever you like best

Eggshell.

It depends if you like shine or not. In my opinion, flat is only acceptable if you plan on never, ever touching your walls, ever. Flat paint dulls on contact and has almost no washability, so if you touch your walls a lot, in about 6 months, they will look pretty bad. You can also use a washable flat, which is different from a normal flat in that it has some washability, but generally won't be as durable as the eggshell finish.

But flat looks better. Shiny paint, even semi-shiny, doesn't look right and calls too much attention to itself.

It depends on who you ask. Decorators and women generally may believe that walls with no sheen look dead. The light has nothing to bounce off of.

I've recently come to like flat (well, technically BM matte not quite as flat as flat) for walls. Eggshell used to be my goto sheen.

For trim satin is required for washability.

The flat/matte is awesome for touching up holes from picture rearranging, so if you tend to tweak wall decor, that is a consideration..

Sheen is not standard, no matter what others say. I regularly use high gloss on my ceilings, flat on trim and anything in between on walls.

Ask yourself these things: Do you want to wash it ? Do you have children or pets ? is the surface perfect or does it have flaws you want to hide a little ?

Most importantly, how does the chosen hue look in different sheens? Dark colors look elegant and sophisticated in high gloss. Pale colors look historic and chalky (in a good way) in dead flat. most color samples are eggshell or satin and don't really tell the whole story about the hue. some colors decide the sheen you choose - for instance, black almost always looks way better and blacker if its a gloss.

If you want your place to look like every other home on the block then by all means paint the ceiling flat white, the trim high gloss brilliant super bleached white and the walls some shade of beige in an eggshell finish.