I assume you mean currently available. You can compare the available carts
here.
Current factors to consider are:
- Price (check current).
- DOA rate (mainly DSOnes).
- Shell quality (whether it fits nicely).
- Available kernels (Wood/ EOS/ YSMenu/ BL2CK/ TWiLight Menu++/ random stock kernel/ multiple).
- Ntrboot support (with switch/ flash youself/ one way flash/ none).
- RTS,
RTG, slowdown (mainly EOS kernel. You might find RTS on your stock kernel, but I wouldn't compare it to EOS).
- SDHC support (mainly just not Original R4).
- Whether you need a hacked system to use it (apart from DS Phat or Lite).
- In game loading speed (slower on DSOne and EZ-Flash Parallel AFAIK).
- The amount of themes made for your kernel. (See
here,
here and
here).
- The sticker that it comes with (or not. But IMO you'd probably want your own).
To add to that guide:
- DSOne seems to be drying up Aliexpress ATM but is still available elsewhere. (Refer to the site
archive.)
- DSOne is bloated and will bulge the cartridge slot slightly (at least on a DS Lite). So keep that in mind.
- If you want "Modern" or "Internal" views (a.k.a. the large icons) in Wood then avoid the "Ace3DS+ Clones". (It's broken on those.)
- I assume that the Ace3DS X would be the same as Ace3DS+ Clones, but I have yet to test it.
- Original R4 shells fit nicely but the bottom most part is thin and might snap. (It doesn't affect use though.)
- EOS has some unique features but lacks cheat descriptions.
And to answer your last question, I think it's the Original R4. As long as you only keep what you're playing and use a modded system. Probably an unpopular opinion though.
Anyway, that's all I've got. Good luck