[Retrogaming] Which old platform or console would be the most worth emulating?

You'd be surprised how many decent titles are available on GBA and NDS, particularly when you're thinking about platformers, metroidvanias, and some military games (less here, I'm mostly thinking Advance Wars). For starters, there are a lot of Castlevanias and Metroids on those platforms. I've played Dawn of Sorrow, Order of Ecclesia, Portrait of Ruin, Circle of the Moon, Harmony of Dissonance and Aria of Sorrow. They mostly rock. Haven't really played the Metroids but we're just scratching the surface here

Currently replaying the God of War games on my steam deck - really enjoying it! Having save states (ability to save anytime/anywhere) for these older games is a gamechanger..

EDIT: I forgot to mention that I am open to turn-based games, maybe similar to JRPGs or CRPGs.
Feel free to recommend me games that can be a gateway series for a platform.

So we're now referring to J2ME as "Pre-Obama Era Cellphone"? :P

For a metroidvania I'll recommend Tomba on the PS1.

For puzzle I;ll recommend Wordtris on the SNES and DOS.

For turn based strategy/RPG I'll suggest Hero's Saga Laevatein Tactics on the DS.

Gamecube has my favorites because it had the best local multiplayer options. Not too many for your needs, but it has a couple strategy titles like Fire Emblem.

MSX and MSX2 has a surprisingly deep library of interesting games and active homebrew community. Many can be played without knowing Japanese, or have translation patches out there. (If you like mid-1980's style games)

Beside the latest consoles (except the switch although Yuzu has been taken down) basically most is just easy to emulate and install, so i don't see why you restrict yourself to just one system.

I like the gamebase versions because then you get 1 easy oversight with screenshots of those games (and thus judge you might like it or not) because of the size of the oldest consoles, the collections aren't that big in size.
Dos has an alternative collection called Exodos.

Still abandonware/emulation is still considered piracy, and i don't know if this clashes with the rules.

There are a lot of good choices there. I would add the Commodore 64 to the list. It has a huge library of games, both its own and solid ports from the Spectrum, Apple II and Atari 8-bit. It has the best sound of any 8 bit machine, to the point where the best C64 game music programmers became legitimate celebrities among the C64 community in the '80s.

I'd also recommend the Atari ST. While most of its best games were ported to the Amiga, it has its own charm and some quite good exclusives. An excellent games machine that too often gets overlooked.

I'd love to play Twisted Metal, Gran Turismo 5, Metal Gear Rising: Revengeance, Race Driver: Grid (2008), Slender: The Arrival, Alien Isolation, Rocksmith 2014, Brutal Legend, Spec Ops: The Line and others I can't name off the top of my head on the PlayStation 3. Who knows if there is emulation for that yet. I know PS1 and PS2 can be emulated but never heard of PS3 or PS4 emulation.

I have PS Vita and a DS Lite. I bought the Vita a few years ago and I love it. In addition to the beautiful yet small Vita library, I can run PS 1 and PSP games. I have about 250 games on all those platforms in my backlog.
A few months ago I got a DS Lite to try out some Nintendo games since I'm not familiar with Nintendo and I'm not impressed at all. The library isn't that big or impressive. I'll finish the games I want to play and then sell it.

SEGA Genesis' "Sink or Swim" alternatively known as "S. S. Lucifer Man overboard"

SNES > GBA > NDS