As commented, both versions are perfectly grammatical.
1: Hong Kong would be a hiking paradise if not for the weather.
Implies either 1a ...would be right now OR 1b ...would be all the time That's to say, only context determines whether it's the current weather that prevents it being a hiking paradise, or that it's never a hiking paradise because of the weather all year round.
2: Hong Kong would have been a hiking paradise if not for the weather.
Implies either 2a ...would have been in the speaker's recent past (i.e. - during his currently ending holiday in HK) OR 2b ...in some hypothetical alternate reality [where HK has different weather].
If speaker is specifically expressing current regret that he couldn't go hiking very recently, option #2 is probably best (because Present Perfect implies strong connection to time of speaking). In all other contexts, I'd suggest sticking with option #1.