As stated, nothing beats the fire wrench. I have yet to have been foiled when the fastener is heated orange-red. They come out, kicking and squealing but they come out. Many people have this perception that you can't work in tight spaces with the torch but I think many people would be surprised what you can do with it. Heating and cutting in confined spaces takes a lot of practice but that's where chopping up beaters before junking them helps to refine that skill.
A sawzall is sometimes needed. As one example, I cut brake disks off several times when nothing else seemed to work. And it's amazingly quick with the right blade. It's often far cheaper in the end and less frustrating to cut something off and replace than to wrestle with and try to save something that's rusted enough to not come out using normal force.
Pipe wrenches work far better than any other binding/biting wrench. The only wrench I know of that gets tighter the harder you push on it.
Air power is almost always better then simple mechanical leverage. Hammering force coupled with compact size means you can get into small spaces with great advantage. Pneumatic wrenches, air hammers/chisels, drills etc are way more powerful for their size than electric.
I have a few tools in non-human proportions that are indispensible; a monsterous slip joint pliers, pipe wrench and pry bar.
I've used just about every penetrating oil and my preference hands down is B'Laster PB.