The great thing about Royal is they have a ton of ships and they cater to different likes. We have been on Oasis, Quantum, Voyager, Freedom and Vision class. It depends on what you like or want as priority. You don't mention if you have kids, which can make a difference.
They all have Main Dining Room, Windjammer (buffet), coffee shops, nightly shows, pools, adult area. They all have rock wall and most have flowrider, sports court (think only vision class is missing these), gym and spa, casino and regular activities in the cruise planner.
Icon Class - Very Large. Just came out so can only comment from
YouTube. Many pools and venues.
Oasis Class - Large Size. This class has the best shows (Aqua shows are unique, aside from Wonder they have a Broadway show, ice show, etc). Aside from included venues they also have great specialty selection for an additional cost. Has things like waterslides and dry slide (aside from Allure), zipline, flowrider. Also unique is the different neighborhood idea with Central Park, Boardwalk, Promenade, etc.
Quantum Class- Large Size. Unique is the addition of North Star (goes above the ship for a bird's view), iFly (flying experience with air pushing you up), Sea Plex that is used for bumper cars and other sports activities. Odyssey is Ultra Quantum with great technology in their shows and other where things are modern. (Just got off this one myself).
Freedom and Voyager Class - Medium Size. Similar with Freedom having an additional pool and a bit larger. Has ice rink, Independence has Grease as Broadway show. Freedom has been amped recently (haven't been on her yet but looks nice). They all have waterslides (not sure about Voyager). These are nice if you want variety of things but not huge. Includes promenade (unlike the next two classes)
Vision and Radiance Class - Smaller size. Less amenities with the extra stuff but still a traditional cruise ship. Centrum instead of Promenade.
It's a personal preference, but I personally love Oasis. Although it's big, it's spread out so doesn't feel crazy. (Going on Wonder so maybe I'll have other comments next month).
To me, they all are very classy beautiful ships with great service and options. We tend to stick with Royal as we love their product, no matter what ship. We even have a Vision class booked in the future so will see how I feel about that one.