Just finished the 1k today and thought I would share my opinions on these. I did Altar, Obelisk and Axis Mundi in pairs and Menhir solo. I thought doing Menhir solo was best because it was so narrow and it was hard to navigate 2 people, especially with all the cracked blocks. Obelisk was the hardest (IMO) because of all the different blocks. There are ice, mystery, monster, cracked, trap and bomb blocks. Axis Mundi kept screwing me up because of one poorly placed vortex block around step 150.
My advice to anyone doing Obelisk/Axis Mundi: When the tower splits around step 100, try to keep the two towers connected as best you can or else you might not have a wide enough platform when the immovable blocks show up.