So here's the whole series (kind of). The numbering makes things confusing.
Wonder Boy
Wonder Boy in Monster Land
Wonder Boy III: Monster Lair
Wonder Boy III: The Dragon's Trap (this is the Master System version. The TG16 version is just called Dragon's Curse and has different (cooler, IMO) graphics. I'd still recommend the Switch remake over the VC releases though.)
Wonder Boy in Monster World
Monster World IV
Wonder Boy 1 is rather unremarkable, and if you've played Adventure Island, you've basically already played it. If you aren't aware, Wonder Boy 1 basically spawned two separate series -- the Wonder Boy series and the Adventure Island series. Here's a fun fact: the TG16 version of The Dragon's Trap (AKA Dragon's Curse) was called Adventure Island in Japan. Makes things even more confusing.
Monster Land is probably the most divisive among modern players but I personally loved it. It plays like a simplified version of Monster World but with a rather strict timer in order to keep the arcade pace. It's challenging but beatable.
Monster Lair is perhaps the blackest sheep in a series full of black sheep. It's a horizontal cute-em-up with an absolutely killer Turbo CD soundtrack. Seriously, just listen to this:
The game is fun but nothing particularly remarkable outside the soundtrack (the arcade version soundtrack isn't nearly as good, but the version on VC is the TCD version thankfully), and if you're a shmup fan you'll probably find it wanting.
The Dragon's Trap is by far the most popular game in the series. It's a very tightly designed metroidvania, though that term isn't quite accurate (Tangent: Dragon's Trap is one of three games that I consider to be forerunners to three separate 2D exploration genres, the others being Metroid and Maze of Galious). While Dragon's Trap isn't my favorite game in the series, it's probably the best game overall and well worth playing. It's a metroidvania with no chaff and a quick pace.
Wonder Boy in Monster World is just a great 2D platformer with some lite metroidvania and RPG elements. Doesn't get nearly the respect it deserves.
Monster World IV is another fan favorite and is very clearly the inspiration for the Shantae series. Personally, I enjoyed the game enough to beat it but it was probably my least favorite in the series after Wonder Boy 1. The hub town is cool, the spritework is charming, and the soundtrack is good, but the levels just aren't very fun.
I recognize that Wii Shop points are a precious commodity now that you can't add anymore. With that in mind, I would say Monster Land and Monster World are the games really worth playing, but maybe look up gameplay of Land to see if it's up your alley. It's the most arcadey of the bunch. Even more than Wonder Boy 1. And then get The Dragon's Trap on Switch if you haven't played it already.
What I probably love most about this series is that every game is extremely different from the last. Only in the Adventure Island branch do the games all play similarly.
