きっと

きっと commits you to the outcome, so it can reassure (きっと大丈夫だいじょうぶ). たぶん leaves room to be wrong, so it comforts no one.

に違ちがいない reasons from evidence to a conclusion (受うかるに違ちがいないよ). きっと needs no evidence and can be pure hope (きっと受うかるよ).

はず is a noun, so it takes なのに, だった and がない (はずなのに). きっと only modifies what follows, so none of those shapes exist (×きっとなのに).

きっと can be pure hope, and sounds encouraging when spoken (きっと大丈夫だいじょうぶだよ). おそらく weighs the evidence, and belongs in writing (おそらく無理むりです).

きっと leaves room to be wrong, your own confident guess (きっと来くるよ). 絶対ぜったい instead leaves none, and refuses かもしれない-style softening.