u can try these mailand pokemon games on 3ds: *omega ruby/alpha sapphire (gen 3) *X/Y (gen 6) *sun/moon/ultra sun/ultra moon (gen 7, i prefer the ultra versions)

I would do firered or leafgreen first

I would would start with fire red or leaf green. But i would also start with platnium or heart gold

I think it would be either something from Gen 4 or Gen 6; Those are when some of the most major shakeups happened. (Physical/special split in Gen 4, Fairy type in Gen 6) All other gens have ignorable additions at best.

(stupid character limit) Playing Gen 4 after Gen 1/2/3 might confuse you from the sudden shift in how moves do damage. Playing Gen 6 after Gen 5 isn't as jarring, but Fairy is a big shakeup that can trip you up if you're not aware.