How i look at it in a fair way.

1) If you want to go by mechanically difficult it would either be Archimonde from HFC, Helya or firelands ragnaros.

2) If you want to go by longest living boss it would be vanilla ragnaros (though he would be the easiest boss by today's standard).

3) If you want to go by the longest boss fight it would be sha of fear, with a 22 minute fight duration. https://youtu.be/Gb1GkVpR6Rs?t=1m50s

4) if you want to go by the amount of wipes. It would be ragnaros at 500+, you can also count yogg here but that would be another story.

Also as for your question

an Mythic Guldan compare to these other bosses in terms of difficulty?

Yes he does, in reality wod/legion by far have the hardest bosses to ever exist in the game if you to go by mechanically difficult. only xavius was disappointing so far ever since mythic difficult was released.

Look at any old vanilla to wotlk videos and compare them to a recent kill video and the skill is not even close to the same.

There were multiple reasons why vanilla/tbc bosses lasted way longer than they should have, and unlike other people i wouldn't consider c'thun a hard boss.

1) bugs, every single vanilla raid had multiple bugs and they usually wouldn't be fixed until months down the road. causing some bosses to last way longer than they should. (in this case c'thun was a very buggy boss)

2) Artificial time added to the raid. Trash would respawn after a while and just simply walking back to the boss it self would easily take 5 minutes. (another good example is c'thun walking back to the boss would take ages)

3) the amount of time a guild spent during vanilla on a single raid was also way less, once again artificially increasing the time a boss is spent alive, they would maybe raid like 8 hours max, IF that. Now we have guilds raiding up to 16 hours a day with, the longest we know is with exorsus, they spent 17 hours in 1 day for EN. That is something that simply never happen in vanilla.