badass-stuck seatpost
Use a really long tube as leverage on the end of a pipe wrench for example (used to grip and twist the seat post (around it's axis). combined with wd40, should get it out, if not the seat post will snap off most likely, if so you can either saw it into pieces using an iron saw blade and take the pieces out individually or as a last resort use chemicals to dissolve the aluminum.

If it's not too stuck you can use the seat for leverage but if it's quite bad you'll destroy the seat (pull out the seat rails) this way. Next step could be to install two big screwdrivers/Allen keys or something similar in place of the seat rails (after having taken off the seat) and tighten it back up again. Then you can use the screw drivers/Allen keys etc. for leverage without damaging the seat post.

Hitting the top of the seat post with a heavy hammer (in axial direction of the seat tube) whilst putting twisting force on the seat post can also help 'loosen' it.

On some seat posts (for example some older SR seat posts) you can use an adjustable wrench to grip the seat post after removing the hardware for mounting the seat. I've tried to illustrate how to place the wrench on the seat post in the image below (forgive the terrible drawing):

Using this method you can potentially get great leverage on the seat post without damaging it so far you'll have to throw it away (the same method can be used with a bench vice (as suggested by @Criggie ) if you have one (grip seat post on the same 2 planes with the bench vice)).

There is a guy on YouTube (RJ TheBikeGuy) who has some good videos on it... example: 
                
            

Here is a video of the 'sawing out the seat post' method: