What if Ravana fights Bhishma? Who would win?
First we would like to state that these two great warriors are from different eras and never faced each other. Ravana was from Treta Yuga, Bhishma was from Dvapara Yuga. In Hindu mythology, power level drops with each yuga. This is just a fictional fight to evaluate their capabilities.
Let us try to compare on five primary grounds.
First, Education:
Bhishma: Teacher is Bṛihaspati and Parshuram
Ravan: Vishrava (His father who is grandson of Lord Brahma), Apart from that some books mentioned he had training from Lord Shiva himself.
Here it appears to be a tie.
Second, Weapons:
Bhishma: He got most of his weapons from Parshuram during his time of education. He never got time to perform any other special Yog (or tapasya) to get any special weapon from deity.
Ravana: Apart from knowledge of weapons he got from his father, he did around 10,000 years of Yog to impress Lord Brahma and got many more celestial weapons. Also he was a devotee of Lord Shiva throughout his lifetime and got many weapons from him like the formidable Chandrahaas.
Here Ravana seems to hold an advantage.
Third, Intelligence:
Bhishma: Was a very intelligent man. He learned all the Vedas. But you know who converted Vedas in the form of shloka or poetry? It was Ravana.
Ravana is one of the (or may be the most) intelligent being in Hindu Mythology.
In all Ravana is the one who won Trilokas. No one in Treta yuga was as powerful to win the Devloka.
Fourth, Warware:
It is said that Ravaṇa was a single Maharathi, but Bhiṣma was slightly more than two Maharathis.
Fifth, Boons (Vardaan):
Bhiṣma was a TriKala Darshi, i.e. he knew what has happened, what is happening and what will happen. So he would know the weakness of Ravaṇa.
Bhiṣma had the boon of “Icchamṛityú” (death at will), however Ravaṇa could be killed by a man. Ravaṇa had the boon of invincibility that he should not be vanquished by Devas, Daityas, Danavas, Yakṣas, Nagas, Gandharvas, Suparnas, Siddhas and other beings. He left out humans and animals as he thought they were too weak.
This is where Bhiṣma could successfully defeat Ravaṇa, as there is a loophole in the boon for Ravana.