ではないか

1. 1Asserts or points out something as obvious or surprising, inviting the listener to recognize it ('Why, it's …!').Ex. 1
2. 2Usually as のではないか, expresses the speaker's tentative conjecture ('Isn't it that…?' / 'I suspect…').Ex. 2

ではないか sounds somewhat stiff and is typical of writing or firm, formal-sounding speech. The rhetorical sense uses falling intonation, while the conjectural sense, usually in the form のではないか, sounds like a softened question or opinion. In casual conversation, じゃないか or んじゃないか is far more common.

• じゃないかN4「じゃないか」 is the colloquial form of 「ではないか」: their uses for confirmation, surprise, invitation, and so on are basically the same, and you can swap them depending on the situation. 「じゃないか」 is casual and mostly used between people who know each other well, while 「ではないか」 is formal and leans toward written language and speeches.
• だろうN5「だろう」 also expresses conjecture and confirmation, but for conjecture you don't need to add 「の」, and it sounds less tactful than 「のではないか」. When used for confirmation it merely seeks the listener's agreement, with no nuance of discovery or surprise, it cannot take the volitional form to make an invitation, and its register is fairly neutral.
• でしょうN5「でしょう」 is the polite form of 「だろう」 and is neutral in register. For conjecture you don't need to add 「の」, and it sounds less tactful than 「のではないか」; its rising-intonation confirmation is softer than 「ではないか」, carries no surprise, and cannot express invitation either.
• ねN5「ね」 is close to 「ではないか」 only when seeking the listener's agreement, but 「ね」 presupposes that both parties already share the information, sounds gentle, and is mostly used in conversation. 「ではないか」 carries a sense of discovery or surprise and sounds stiff, and it has conjecture and call-to-action uses that 「ね」 lacks.
• っけN3「っけ」 is used to confirm something you can't quite remember ('what was it, again?') or to reminisce about the past with nostalgia. 「ではないか」 is for when you have formed your own judgment and want the listener's agreement, or for expressing conjecture or a call to action; 「っけ」 is very casual while 「ではないか」 leans toward written style, so the two cannot be interchanged.

じゃないか is simply the colloquial contraction of ではないか. だろう and でしょう state a conjecture more directly, while ね softly seeks agreement without the rhetorical, assertive tone. っけ asks the listener to confirm something the speaker has forgotten, not something obvious.