Son has the highest top level performance. Consistent in the PL and won games single-handedly for Spurs in the CL. He would go down as the best Asian footballer if he played for Bayern or Liverpool. He would easily fit their style and I think he is on the same level as Mane and a better player than Coman.
Nakata was very influential in ushering future generations of Asians into Europe but I don't think he was that great a player. He was highly marketed by the Japanese and immensely popular. The hype surrounding him was also boosted by Japan being the host country for WC2002. I remember in the early 00s he was frequently brought up in conversations alongside the likes of Zidane, Figo, Beckham. He was nowhere near that level. He was a bit part player that was sold for record fee. Personally, I think Roma completely fleeced Parma in that deal.
I agree with you on Nakata. He was a good passing 8 with a strong shot, but seemed to become very slow quite quickly and was very inconsistent defensively for that position. After a good first season and a half at Perugia, i don't remember him ever coming close to replicating that form.
I watched him closely when he came to Roma as i was following Alenichev at the time who went to Perugia as part of the swap, a player i rated highly so was curious to see what Nakata offered that was better. Capello wanted a sturdier, more box to box central midfield player than Alenichev ( tiki-taki'ish supporting playmaker that didn't defend well) for his system and Nakata went straight into the team in mid-season, but didn't cope well at all with the demands of Capello.
he was quickly moved on (though probably still a cult legend for his vital goal against Juventus at a key moment during their league winning season) and played bad most of the time at Parma too, though to be fair they weren't a stable club by that time. I remember lots of questions about his fitness and dedication by this point.
I know he was a big star in Japan at the time, but overall i found him a quite sluggish player that needed protection and space, classy but not very effective. He didn't seem as mobile, or good on the ball as the later Japanese creative players mentioned earlier like Kagawa, Honda, Endo etc and those later teams played a more sophisticated style that could have been quite formidable with better forwards available.