I know it's a huge topic, but I can't understand the differences between all this ways to say oneself, and by extension yourself, himself, herself ...
If anyone can help me to have a picture of the uses for oneself, it will be very helpful (and i'm pretty confident that I'm not alone in this situation :D)
Thank you :)
Although the words you listed are all semantically related, they each have quite distinct grammatical roles in the typical usage (with little overlapping) that you never want to miss.
自分 is a standalone pronoun that substitutes the same referent (noun) that appears in the context. While it certainly can be categorized as "reflexive pronoun", its chief use cases are sentences which have multiple actions that involve the same participant. Don't expect too much for it to really translate oneself or such in English, and vice versa. Most of the times they have better lexical expressions; try to look up those parallel corpora.
彼は自分を笑った者たちを許さない。
He does not forgive those who laughed at him (*himself).He washed himself.
彼は体（× 自分）を洗った。
It also represents various meanings usually translated with adverbs in English:
自分で by oneself; personally; alone
自分から on its own will; spontaneously
Sometimes, 自分 appears without an apparent antecedent. In this case, it means general "you". Japanese usually omits such a pronoun, but it is required, like above, when it has references to the same agent. (Natural translation may become "me/myself" or "him(self)" and such according to the context.)
視点を持て Have a perspective.
自分の視点を持て Have your own perspective.
As I said "standalone", 自分 does not form compounds with other nouns. It is only accompanied by particles or affixes (affixable words).
自分たち oneselves (plural)
自分らしさ one's own style; being oneself
自分どうし (see what it means)
己【おのれ】 is simply an archaic and/or solemn variant of 自分.
Meanwhile, 自身 is a suffix, always attached to another noun to add meanings like the very —; — oneself; — per se etc. The word must be put right after the noun, though the resulted translation may not need to be. Sometimes it is just for extra emphasis; other times not:
ボス自身は弱いが、シールドを壊さないと攻撃が通らない。
The boss is fragile in itself, but you can't damage it before shutting down the shield.
If you combine 自分 with 自身, you get 自分自身.
Note that both 自分 and 自身 are mainly for animate nouns. In most cases you should use 自体 instead of 自身 for the "per se" sense. There is no exact replacement of 自分 for inanimate nouns, so we usually use それ or repeating the noun in such context.
紙幣はそれ自体に価値がない。
Banknotes do not have value on their own.
自【みずか】ら is a synonym of the suffix 自身 with a couple of reservations: (1) there is no 自分 + 自ら; (2) usable with animate and inanimate alike.
自己 is almost always used as prefix to make compounds that means self- or auto-. It has a large number of derivatives:
自己紹介 self-introduction
自己修復材料 self-healing materials
自己免疫 autoimmunity
自己愛 narcissism
Lastly, there is another prefix 自-, which is either (a) same with 自己 exclusively for building two- or four-kanji idioms; (b) of one's; or (c) your own group, "friend" as opposed to "foe", or "we" to "they".
自業自得 You reap what you sow.
自動詞 intransitive verb
自チーム one's team
自陣 own base, own half, home territory
自文化中心主義 ethnocentrism
自分 is, besides the above, used for true noun for (one's) self. It is the simplest word that kids would use for this sense. Has some compounds:
自分勝手 selfish, self-righteous
自分探し finding yourself
自身 can also be used apparently alone in bookish styles, which technically should be taken as omission of the previous noun. Not as truly independent to form compounds. However, compared to 自分, it focuses more on external properties, such as: one's past, one's life, one's deed, one's belonging etc (=自分の身, 我が身). Also applies to 自ら.
ボルトは自身／自らの（=彼自身の=自分の持つ）世界記録を更新した。
Bolt broke his own world record.
自己 is rarely used as noun except for self as a psychological term. Otherwise you can safely forget about it in modern writing.
自力 is literally "one's own capacity". Almost only used in the collocation 自力で "by oneself (without others' help)" = 自分（の力）で.
自力救済 self-help (legal term)
自力更生 self-reliance (Chinese slogan, lit. "revitalize on our own")
自分 is sometimes a first-person pronoun I, but also dialectal informal you in Kansai.
己 is also informal you in some (mostly Western) dialects.
自ら can be used alone as an adverb: with one's own hands/initiative; at first hand.
Some situations can be expressed with multiple options.
The CEO supervise the project in person.
= 社長が自分で計画の音頭を取る。 (pronoun + case)
= 社長自身が計画の音頭を取る。 (suffix)
= 社長が自身で計画の音頭を取る。 ((pseudo-)noun + case)
= 社長自らが計画の音頭を取る。 (suffix)
= 社長が自ら計画の音頭を取る。 (adverb)× 社長自分が計画の音頭を取る。 (not affix)
× 社長が自分計画の音頭を取る。 (not bare or as affix)
× 社長が自身計画の音頭を取る。 (not bare or as affix)
Bonus:
This might not be the most complete answer, but I might be able to give a general picture for what they are used for. I'll give a summary of what I have read, mostly which comes from goo.ne.
自分 - This refers to the "self" in both the concrete and the abstract. For example 「自分の体」refers to your physical body. 「自分の考え」is also perfectly valid.
自分のことは、自分で決めろ。
Decide for yourself.
自分自身 - This creates emphasis by putting two words with similar meanings together. It emphasizes that "yourself" as an individual.
自分自身を大切にしなければならない
You must cherish yourself.
自己 - This "self" mostly refers to the abstract. The example given was 「自己の信念を貫く」, "To stick to your own beliefs". It also appears in a lot of compounds like 自己紹介 and 自己犠牲.
己 - This sounds very archaic. You'll find this in old proverbs as well as cheesy dramas about old Japan. It has almost the same meaning as 自分.
全ては、己の弱さを認めた時に始まる
Everything starts from realizing your own weaknesses. (From JOJO)
自身 - Although this has a lot of overlap with 自分, the difference is that 自身 can also be used to refer to inanimate objects. It can mean the equivalent to そのもの. 自分 can only refer to "self" as something with volition. 自身 can also be used to emphasis the noun before it. For example:
彼女自身の生き方。
A lifestyle of her own.
自ら - This has the same meaning as 自分, but it can also refer to groups of people. In my (possibly wrong) opinion, it also sounds a bit stiff and formal.
彼は自らの力で勝った。
He won with his own power.
