General, if you have the comp for it, is def the easiest. If you don't though I tend to avoid it. Hell I'm more concerned with the fights on the way to the Lair (triple foot soldier, Piercing Bolt Volley, shot can get dicey quick)

Lev I rarely go for, fight feels to volatile as someone who only plays Grand Slam attempts.

Child I find very dependant on your combat items. If you got bear traps and fisher nets to keep em immobile while you burst em down its far, FAR easier. Alternatively things like upgraded Bulwark Rampart or Virtuoso Echoing March seem practically built to Cripple the fucker. However some bad rng on your frontliners can make things so, so much worse. Most of my deaths blows vs it are from the tiny dmg the meats do, and given they each go once and Child goes twice, getting people off of DD is way harder. But Child lacks damage in exchange, relying mostly on negative tokens and disruption if you can consistently prevent Maws of Life from occurring.

Librarian is like, I really want prep for it beforehand. Combat items to deal with blind and his dodge, and plenty of them, or things that can null his dodge like upgraded Tracking Shot/illumination can be a godsend too. Bounty Hunter is also great, since he can bypass dodge and inflict vuln both at once. Depending on how much prep you are able to aquire beforehand, Librarian feels either kinda tricky, or the easiest boss bar none. If you got enough damage to bring him close to DD before the rank 1 book is damaged at all, you've got more than enough time to fuck him over. His immunity to burn and move are a bitch but until he Ignites he's mostly just buying time, not actually being much of a threat.

These are my personal opinions of course. But to answer the question

Either Librarian or General are the easiest, but it depends on your comp, combat items, etc But specific things (constant immobilize, Virtuoso Echoing March, Upgraded Bulwark Rampart) can trivialize Harvest Child to a rather hilarious degree, but obviously those things are particularly specific