True
I’m supposing best = an optimal balance between speed, cost, and convenience.
2 hours per leg, or 2 hours round trip?
That'll be (just under) two hours each way on the bus: but of course you have transit time to/from the Dubai bus station.
Car should be faster, but it all depends where you are in Dubai. Traffic can be nasty.
The attractive feature of the bus is comfort, low-stress, and of course the price - under USD15 round-trip. It drops you in the central transport hub of Abu Dhabi, from where you can start spending the saved cash on taxis...
Self drive is easy on the motorway, but can get a little confusing in city traffic. Driving standards are generally good, though there are very definitely nutters around - and other drivers aren't always helpful/forgiving if you get yourself badly positioned. Speed cameras are a problem for newby drivers, it's easy to drift above the limit. Locals know when and where to slow down. In the city parking can be a faff. Rental, tolls, gas and parking do mount up.
A car + driver for the day would be the "best" solution, freeing you up for being a tourist in Abu Dhabi. But it's likely to be expensive. You could ask around once you are in Dubai. Your concierge should be good for giving you an idea of the absolute ceiling to prices