^Ahhh, hold on. I want to tip it to Super Star's favor. I just played through all of these again (Super Star, Super Star Ultra, and RtDL) like a week ago, so they're pretty fresh in my mind. Super Star Ultra is clearly the best of the bunch, with RtDL coming in at the bottom of the list for me. You're going to have a blast with Kirby's abilities no matter what you choose if you're only experienced with Adventure.
It's probably more important for me to explain why I didn't like RtDL as much since it's relatively new. Return to Dream Land has the most evolved copy abilities yet, with movesets most similar to those in the Super Star games. The bosses, music, and visuals in RtDL are also really nice.
It's brought down by a few things, though:
I'm not the biggest fan of collecting random cogs in every stage. This is how the game unlocks its content (copy rooms, challenges, mini-games). You'll find anywhere between three to five (six?) in a stage. Most of them are either inconspicuously tucked away or inside a puzzle room. Some are dropped by mini-bosses. I don't like collection in platformers; only a handful of games do it right. Playing RtDL solo, you're almost always required to take a specific ability to solve a puzzle. I hate this because you won't be able to just go through stages how you want to the first time around, taking away that freedom of choice I've always associated with Kirby.
The level design in RtDL is also very simple and somewhat uninspired. It's almost infuriatingly simple. So many puzzles feel like baby puzzles. A couple of stages have a few neat ideas, but it's honestly rare for me to play a Nintendo platformer and be painfully aware of how underwhelmed I am at stage gimmicks. The traditional platforming Kirby games have never been particularly strong at this, but RtDL really disappoints me in this area. Some ideas and motifs are recycled from Adventure and Super Star. I don't know if it's just me feeling this way about the stages, but it definitely is a sore spot.
An EX mode unlocks that makes the game a little harder, but you're already God with half the abilities in the game so it's barely any more challenging. You have less life, there are more enemies (I think), and bosses are a bit harder. I'm glad that it's there, but it isn't hard enough to be considered a hard mode by any means. Arena and True Arena are great, though.
Oh, challenges: they're neat ideas but are poorly executed. You won't really return to them because each one is very obvious and very linear. You'll maximize scoring potential in no time.
You can play with others, but the multiplayer is not the game's strong suit. Kirby remains the star, so any other players who aren't Kirby get shafted. I actually don't know if the second Kirby player gets shafted, too. The camera will follow the main Kirby, and the game is over if that Kirby dies.
There are new things called Super Abilities that are kind of cute at first but eventually wreck any pace the game has going for it. You basically just plow through an entire section of the level until you run into a dimensional rift that leads you to a chase stage and cog mini-boss. Some people might like this, but most of the time it felt like pointless filler to me. I did laugh out loud at a couple cute moments. I just don't think a game that's already super easy should demand these stress-relief sections.
I don't mean to sound down on RtDL as it's still a decent-to-good game, but there is a clear disparity between the awesome/broken capability of Kirby and everything else designed around him.
As for Super Star, it's much harder to describe. It's less of a cohesive package (maybe less traditional is a better phrase), being a bunch of smaller, unique games rather than a single, sprawling adventure, but I think it's way better. Each "game" is totally different. Some are easier than others. Some completely change up goals and mechanics. Some are mini-game diversions that will see less of your time. Since you don't seem to have played it, I don't want to be too specific. Just know that the way the game is laid out makes it easier to play in shorter, more specific bursts. For example, it's much more convenient to modify your playthroughs by choosing to use helpers or not or only using one ability or taking no damage etc. The game definitely has a huge fan following for a reason.
If possible, go with Super Star Ultra. Ultra still has everything in Super Star; it just adds more content to the mix, including some standouts: Revenge of the King is basically Spring Breeze EX; Meta Knightmare Ultra takes Meta Knight through a speedrunner's abbreviated version of the whole game; and Helper to Hero is the Arena where you can play as Helpers. It also has more functional viewing space (without the UI cluttering up 1/4 of the screen) and is portable.