Finished all four of these this year and liked them all. If you're deciding which one to buy, here's some thoughts I had as I went through the franchise:

Risky's Revenge: Might be a little too primitive to recommend to most people. A short but solid GBA-era metroidvania. It's not awful, but we have so many metroidvanias with modern QoL that this one only feels worth it for super-fans.

Pirates curse: Tied for fave for me. The cleaned up interface, gorgeous areas and improved combat and map design make this one a safe go-to to recommend to anyone. The best way to describe it is that it does everything well enough, nothing really bad, and is probably the "safest" game in that nothing it does is too wild. The core elements of it are all pretty solid.

1/2 Genie hero: Probably my favorite. replaces the metroidvania map with more precision platforming segments. Best level design in the franchise by far. Stages are wild: everything from cities under siege, magic carpet armadas, gorgeous waterfalls, and colorful deserts. Best transformation mechanics in the franchise with dozens of them that enable different gameplay segments (for example, the harpy always falling and needing to flap to stay afloat - and going through spike mazes with no floor).

Seven Sirens: Eh. Returns to metroidvania form and does that aspect well. Very polished. I loved the exploration. My big issue was the stage designs all felt like different variations of "colored bricks in the sewers". That was hard coming fresh off half genie hero, where every stage was exploding with action and color.
Even worse though, it got rid of almost all the transformations so segments from previous games like spike mazes and air glide is just abandoned completely and they kind of scrap the platforming in this one. You just kind of walk from place to place. I really wish they hadn't gotten rid of whistle/warp dance because I spent a lot of time backtracking. Boss design wasn't great. Got rid of all vertical mobility until you're at 90% story completed. Really weird game because its like 1 step forward 2 steps back. Despite all that, it's a great game. I just think the mechanical and design decisions mostly went the wrong way.

They're all great, but Risky's Revenge kind of depends on your tolerance level for what it is (barebones GBA era metroidvania). Everything else is a solid recommend.

The utterly terrible, static map is the sole reason this game is so unnecessarily tedious and ultimately made me stop.

Its this inaccurate map that does not log important discovered details NOR gives you the possibility to do it yourself, that made me finally stop.

Because this game was once developed for Gameboy Advance they had to save on data space. Becasue of this elements in the levels are repeating a lot. For example certain cave entrances look the same no matter how important the contents inside are.

So if you find a new ability that lets you traverse new areas or hindrances be prepared to travel between these unmarked caes over and over again to find the one that has the next necessary Upgrade or Item in it.

Its quite a shame and something they should probably have attempted to fix at some point.

The Artstyle is actualy really nice but that doesn't help If I am tired of seeing them over and over again for no good reason.