[quote]fossil fuel < fission < fusion < antimatter < subatomic < exotic particles < subspace < singularity/gravity < ???[/quote]
[@Forsythe]
I wanted to note this before but that list of energy sources is not quite in order.

Heat energy is generally what you can describe as fossil fuels, steam engines or even exotic plasma reactors are more or less this (unless they use electromagnetic energies). The method how they generate heat can vary but their yield is limited by the amount of heat energy they can gain.

Technically nuclear reactors are the same in mechanism but for obvious reasons we can talk about it separately. Nuclear fission and fusion roughly offers the same power yield and it's usually just the matter of design and choice of fuel. They are limited to a few percentage of mass energy. That's still far better than usually any classic heat engine, of course.

Subatomic and exotic particles may allow more efficient mass energy extraction so they could be higher up.

Matter-antimatter annihilation reactors can utilize up to 100% mass energy, though less efficient designs or simple uncontrolled pair annihilations are around 10% effective mass energy, IIRC. The best nuclear reactors approach or surpass the worst antimatter reactors.

Black holes as power source are classically not superior to nuclear reactors in output. Though it depends on the characteristics of the black hole. There's also the fact you need advanced gravity manipulation to make black holes portable and then you probably can alter the characteristics of the blavk hole, too. Ideally black holes can be close to antimatter reactors in energy efficiency (suppose you can freely alter how much energy the black hole releases) while your fuel sits in a compact spot and you can "recharge" it by just feeding any junk to it.

Subspace energy generation is unquantifiable since it's a fantastic world with different sets of laws. It can have power yields of whatever you find convenient.

The absolute top dog of known power generation methods is vacuum itself and how! Vacuum energy is just huge and it's present everywhere. While telling power output is impossible since no known mechanisms exist the power density of vacuum is estimated around 10¹¹³ joules per cubic meter! Well, according to one interpretation. There are five most popular ones ranging from zero to infinite. This is also called the zero point energy and if you watched Stargate you may be familiar with ZPMs.

That's it for now. Hopefully my explanations were helpful.