Hi Holly and welcome to the forum!
Well firstly congratulations on booking what will be a fantastic holiday!
All the restaurants serve Greek food, you wont get a bad meal in Arillas in my opinion, part of the fun of going somewhere new is to sample all or nearly all of the restaurants, they have boards outside with their specials on, see what takes your fancy and whichever one you choose im sure you'll love.
The next night try a different one, then you can find out regarding your own tastes and views, which is your favourite restaurant/s, then post all about it on your return!
My personal favourites are Thalassa, The Captains, Brouklis and Costas on the Beach.
Have a great holiday
!