My guess, from a position of almost total ignorance, is motorbike (esp if Wyre83 is driving!). Next would be car, then bus. There are no direct flights (so it means going via Manila) and no direct ferries (I think that would mean going to Iloilo (IF such a ferry exists) and then a ferry to Bacolod).
This reminds me of a local joke back home, which can be adapted for anywhere: "Can you please tell me the quickest way to the local hospital" ... "Yes, go to (insert the name of your nearest rough pub) and p*ss in someone's pint".
The fastest way is via Mabinay and Kabankalan. Going via San Carlos is a more pleasant drive, and takes about 1.5 hours longer.
I make the drive almost monthly. Mabinay, Kabankalan is the fastest but also annoying. Once outside of Kabankalan you will face road work until you get through Hinagaran and some little towns north of there. The problem is, that Negros Occidental does not have traffic people out there to control the one lane, so you could be stuck waiting for some time as the others once they start are like ants to a picnic. Also a lot of tryke traffic on the main road which is mostly one lane and then the sugar cane trucks. Once you make it to Bago the next slow down comes in Sumag and then the real fun begins you arrive in Bacolod, home to the worst drivers on this entire island. Good luck.
Update, Just returned from Bacolod today. Felt a little adventurous so took the Don Salvador Benedicto, San Carlos route. Once you enter Negros Oriental the norm is massive road construction, drainage work until you reach Bindoy. It is terrible, frequent stops because only one way, and when you get to go a complete mud bath for your auto or whatever conveyance you use. When you are not in a construction area you are going over areas that are being readied for construction once they finish the one they are working on. Take the Mabinay route if at all possible. Me? If for some reason the Bais to Mabinay or Mabinay to Kabankalan was closed, I would drive south through Bayawan and then up versus going up to San Carlos, even thought it adds an extra 1 to 2 hours of travel time. I might give it another try in a year, but not before.
"I might give it another try in a year, but not before." Or in two years bcs same situation existed January last year when I came back from Bacolod.
With a car certainly the construction sites are depressing. I recently went with the bike to Bacolod and back the next day and took the San Carlos route on the way back. I think that if you go by bike and aren't in a hurry San Carlos is by far preferable. It's a much more scenic route, less traffic (especially less trucks), and the one way construction sites are not an issue with a bike - most of the time you can sneak through somehow even with oncoming traffic or at the very least skip to the head of the queue. And the gravel sections make the ride more interesting. Next time I'll go this or a similar route again. Most notably I went to Bacolod on a Sunday through Kabankalan and back through San Carlos on Monday, but still traffic on the way back was less.