I feel most of them are a little ''meh'' for a level 20 achievement. Each capstone that give ''a little more use of this and that when you roll initiative'' disapointing; there's nothing new, is just a little more resource at a level where you should have enough. Thinkin' of you bards and sorcerers.
I love the barbarian one because it really represent the idea of being more than ''human'' on level 20.
I dont like the druid's one, not because is not powerful, but because it looks like it was made with only one archetype in mind. In my games (should a druid reach level 20...so not likely), I give a special capstone to each archetype, much like paladins. Sometime I wish capstones were all decided by the archetype instead of by the class.
Archdruid (Land):
At the end of a long rest, you automatically add the Circle
spells of the land where you finished the rest. You lose those
extra spells when you take a long rest in another land.
Master Shifter (Moon):
You can use Wildshape a unlimited number of time.
Night Watcher (Dream)
You add the spells Find the Path, Forbiddance and Heroe's
Feast to the available spells for your Walker in Dream
feature. Those spell can be used at the end of a long rest only
but follow the same rules otherwise.
Spirit Friend (Shepherd)
Your summoned creatures have resistance against nonmagical
attacks.
Death Bloom (Spore)
When your zombie die because of damage from an attack,
it deals 3d6 Poison damage to every creature within 5 feet.
Creatures that dies because of this damage are automatically
raised as a Zombie with 1 hit point.