As cajungirl notes, it also depends on how set you are on a 7 day cruise, or a shorter length. I booked a friend of mine on the Conquest this past April. He had a wonderful time.
The Elation and Ecstasy are older ships, but still a good cruise value. They just don't have as many new "things" as the Conquest.
In my view, for a first time cruiser you can't beat the Conquest. It will give you all that a cruise can offer and I feel you will be impressed.