If money completely does not matter I think Construction might be the fastest.
I seem to remember a ridiculous thread on RSOF where a guy laid out how to get 200M construction XP in like less than 24 hours played time, but it consumes a lot of GP.
Even if you do Con with Oak Larders (which obviously isn't the fastest) I think you get at least 300k XP an hour. Getting 99 that way costs like 20M or less if you use SC hammers.
Cooking you can get 400k XP an hour doing things like Wine, and these days there are probably other really fast methods that don't totally burn your cash.
I remember smithing was fast when I was doing Addy Plates, but I don't remember the XP an hour. I don't think it was as fast as Cooking and Con, though.
Herb I wouldn't think is near the fastest, either. I would assume you get 200-300k XP an hour in that and any pot you do is going to lose you a fair bit of money.
Tl;dr I think Con is the fastest, probably followed by cooking.