Instead of using the word "best", it would be better to use terms like fastest, cheapest, most comfortable, etc.
Anyway, here's another option...
Belize VIP Transfers & Tours is advertising in the local media that they have added Merida as a new destination. No fares were included in the ad.http://www.belizetransfers.com/en
Chetumal has an airport. Maybe you could search and see if there are any flights between Chetumal & Merida.
Are you aware of the Maya Island flights to Cancun. Then maybe flight or bus to Merida?