Of these menu 'side' options, which one are you most likely to choose?

I have two definite choices, and would really like both: Greek Salad (with Feta Cheese, of course) and Potato au Gratin. I'm a protein, starch and salad guy.

Salad. Celery soup sounds awful

Potato Gratin - it goes with lots of things.

Greek salad with 'Moussaka'.

Celery soup with a chicken salad sandwich.

Olives with breads & dips [tzatsiki' baba ganoche e.t.c.]

Saffron rice with Chicken curry.

Potato gratin with pork chops or steak.

Potato gratin ..

Chances are the rice is not a "real" Saffron rice so that would not be an option.

I would not consider Marinated Olives a "side" they may be part of an appetizer plate to go with some roasted veggies, cheese maybe a dip of some sort but most definitely not a side by them selves.

Soup might be a possibility depending on what the main course is.

Salad would probably be my second choice, If I am at a Greek Restaurant it might be good.

Most other restaurants I think I would opt for the potatoes.

Twice baked potatoes.

probably the rice

It depends on the main course... for example, if the main course was lasagne, you wouldn’t choose potatoes as a side dish. If the main course was beef stew, you probably wouldn’t choose celery soup.

Menus have to be planned in context, as befits the occasion, time of year, diners, etc.

It would depend on what the main part of the meal was.