BoltBus
BoltBus is great, I take them whenever I can book a ticket. Power outlets make a huge difference in my trip! Having WiFi isn't bad, but my tethered phone tends to be more reliable.
Megabus also has power outlets, but the buses are always late, by 1-4 hours.
Fung Wah and Lucky Star are much more convenient (between the two of them, there are buses leaving every hour) but they don't have power outlets.