What does everyone think has the highest energy density per pound? for whatever that answer may be would be the best choice for a mobile device. For more energy in less weight means more efficiency.
From there we need to then ask what engine or ( conversion device ) has the highest efficiency at converting the stored energy to rotational or accelerating energy. For when we have the answer to A and B we then can ask C how can we make A and B work together, for then and only then I feel we can get the answer that we need for the future of mobile propulsion.
Honestly there is no single best of any. As far as transportation and its related fuels go there are too many applications and variables that have far too differing sets of working conditions.
What would work best for a short range one or two person grocery getter that sees only a few tens of miles per day at most will not work in a long haul over the road rig that has to carry multiple tens of tons many hundreds of miles every day.
Just the same as a small in town paved street orientated vehicle will not work in any practical fashion on any realistic off road and rural travel application or driving conditions either. The off road capable vehicle will be able to do the paved road city work but at a loss of fuel efficiency being it was not designed to work in high levels of stop and go traffic.
This also carries over into rail applications in some aspects as well. Inner city public transport is best suited to electric drive via a dedicated electrical grid but trans continental freight hauling is not effective or practical with electric power. The distances and terrain are just too far and too extreme for the electrical infrastructures to work with not to mention the power levels that freight trains require. One average freight train can easily have three engine units putting out over 10,000 peak hp between them just for relatively flat land operating conditions alone.
Lastly with any form of electric plug and charge or power by wire vehicle of any sort there is still that problem of where does that electrical power come from. What is the fuel behind the electricity so to speak.
There is no one best fuel or one best vehicle or one best anything in this application. Whats is highly efficient and successful in its intended environment will be a complete failure or highly impractical in another.
(Scead quit arguing with idiots. They just bring you down to their level and beat you with experience every time. Its a sad but
irrefutable fact!)