I think the hardest is Chen Hong in recent years, right? He has very powerful smashes in his games. Especially in his post-China team days, llike the one in WC2006 against Taufik. His strongest smash was 303km/h and his average smashes in that game was around 270km +, which is very incredible ~ What you guys reckon ?
CH was a big smasher, but he was still in the china squad for WC2006. LD CY and CH are all huge smashers, and surprisingly CH used the head light NS8000 in WC2006. Now that he's no longer part of the China team, LD still has the biggest smash, but I'd have to say that BCL's smash is sometimes more dangerous because he plays it only when it'll get him the point, and it's very steep cause he's so tall.
I believe Fu Haifeng had one of the fastest recorded smashes a few years ago. But that's doubles, and you're all talking about singles players. Probably LD in singles though.