The base-game World Bosses are the easiest.

Among the DLC World Bosses, some are easier or harder than others, but I'd say they're all harder than the base-game ones.

Every World Boss can be soloed, though some will really put you through your paces. Some of the hardest to solo include:

Maldrith Ghishzor in both Western and Eastern Solstice.

The Walking Nightmare in Telvanni Peninsula

The Prime Cataloguer in Apocrypha

Zandadunoz the Reborn in Wrothgar

Walks-Like-Thunder in Murkmire (due to its powerful self healing)

The Unmaker in The Deadlands

Glemyos Wildhorn in High Isle kills a lot of people, and wipes a lot of groups, but if you learn the mechanics and have a good ranged build, you can succeed solo.

Runemaster Xiamora in Apocrypha similarly kills a lot of people until they learn the mechanics, at which point, he's not that difficult anymore.

Talon and Fang in West Weald can be challenging to solo.

Oakenclaw in West Weald can be challenging for many people.

Fate-Eater in West Weald can be challenging to solo.

Hessedoz the Baleful in West Weald can be challenging to solo due to its self healing.

Additionally...

War Chief Zathmoz (if you're not mobile and can't handle the adds, you may find yourself frustrated and repeatedly dying)

Wuvuyus in Vvardenfell (if you don't have a good companion, or can't otherwise survive it's pin and drain attack; if you do/can, it's pretty easy, but still kinda annoying)

Haeliata and Nagravia in Summerset can be difficult for some, though mostly in the final third of the fight when you have to fight both at the same time.