This is a really hard question for me to answer. So I'm going to go with a bit of a catch-all.

I really like the GBA. But the DS can play DS and GBA titles. But the 3DS is great too and can play all 3 and then some.

I guess the 3DS. There's just so much that can be packed into it. I haven't played mine in a while. I wish it was easier to install a capture card in them. There was something special about the games from the GBA to 3DS. The limitations of the hardware had developers experimenting, and we ended up with so many interesting titles. If you factor in the couple n64 games ported over to the 3DS, it's just so much nostalgia in one handheld.

If I had to go with a runner-up, I think it would be my series X. Being able to play so many backward compatible games is also super nice.

If I truly had to pick single consoles only, with only the games designed for that specific console. I would probably go with the 3ds and the xbox 360. The 3DO or Dreamcast would come after as they had so many unique titles.

I never got to play the ps1, ps2, or psp and vita. So I've been curious what I've been missing out on over the years.