Ghost vs Flash

Flash is better in most cases, but there are some champions that ghost works better on. I know hecarim for example takes ghost almost every time.

yes

Depends...

one goes over walls the other doesn't, one let's you dodge spells at the very last second the other doesn't, one let's you chase enemies more effectively than the other, depending on the situation one let's you disengage from fights better or engage better

It depends on your playstyle. Flash allows you to do more fancy plays but the heavy CD means that you are more limited. Ghost is useful to cover grand distances and it has lower CD which makes it a tad more versatile but it offers nothing that items can not offer you already. (the effect to go though units in on a couple of items and for ms you have deadman or switnees boots)

Personally I take ghost on tanks if I already have an independent dash on my quit (something like sejuani Q but not amumu Q) and take flash otherwise

flash has the invaluable trait of being a BLINK. not a dash. so it allows a champion to separate themselves from an enemy champ by going over a wall instantly.

you can't really put a price on that.

the only champs who should be running ghost OVER flash are champs who either benefit from pure MS like hecarim or who really need to just run at someone and stick to them. volibear, udyr, olaf. basically juggernautish champions. I think you could situationally run it on any juggernaut and justify it. garen, darius, illaoi, mordekaiser, skarner, etc.

i mean a lot of those are better with flash still just because once they get to you they can deal their damage easy, but others have sustained damage so like...olaf doesnt do a combo once he gets to you. he just keeps wittling you down. Garen can flash Q and then do his combo since you are silenced etc.

still, in almost all situations flash is the choice. it has been meta since its inception for a reason.