Hi marie and wecome to Cruise Chat.
The most desirable place to be on any deck is considered to be the middle of the deck, neither front nor back, for the least amount of movement in the cabin.
That said, there are exceptions. If you want a room with a balcony, often the balconies at the back of the deck are larger than on the sides. If you are prone to motion sensitivity, being lower down on the ship is better than being higher up. If you want a balcony, however, you have to be a bit higher up because the rooms on the lower decks don't have balconies.
It's best to talk to your travel agent about your requirements for a room. S/he can then look at which actual rooms are available and put you in the best within the price range you want to pay.