Prowl VS Starscream
Both are second in command. Prowl is a great strategist, Starsscream an idiot and coward. So my bet is on Prowl, unless Starsscream manages to be to unpredictable for Prowl.
Prowl VS Onslaught
Strategist VS strategist. In a dual, Onslaught probably wins. But as strategists coordinating the fights it could be interesting.
Grimlock VS Megatron
Clear victory for Grimlock so far. Would have liked to see him kick Megatrons exhaust more often.
Grimlock VS Galvatron
Grimlock did appear to throw Megatron around with ease, would be interesting to see how he would fare against Galvatron.
Blaster VS Soundwave
Would like to see more of this. If both brought their cassettes with them Soundwave would probably win because he has more casettes (Rumble, Frenzy, Laserbeak, Buzzsaw, Ravage, Ratbat, Slugfest, Overkill) then Blaster (Rewind, Frenzy, Ramhorn, Steeljaw).
Now on to the heavy stuff...
Dinobots VS Devastator
They have been up against each other 2 times without a clear victory for either side. In Heavy Metal War each side delivered a beating and although it appeared as if the Dinobots got kicked they did got up again with ease. When the other Autobots arrived it appeard the fight was in a stalemate with both sides facing each other. It appeared as now neither the Dinobots nor Devastator dared to attack the other.
The other fight was in
TF The Movie. Although this was not a fair fight as Snarl was missing. Devastator did knock down Grimlock, Sludge and Swoop but Slag knocked down Devastator with a single hit!
Afterwards the Dinobots appeard uninjured but the constructicons retreated. Perhaps they beaten Devastator offscreen?
We have seen the Dinobots attack one by one but if they attacked all at once I think they could overpower Devastator. I think Devastator is to slow and stupid to concentrate on more then 1 enemy at a time. He might also not be strong enough to cope with a combined attack.
Omega Supreme VS Devastator
That fight in 'The Secret of Omega Supreme' makes me want more.

Probably a clear victory for Omega.
Dinobots VS Predacons
I guess the Dinobots would easily beat the induvidual Predacons (they are bigger and are known to be able to take punishment) but when combining to Predaking I think Predaking might eventually win.
Induvidually:
Grimlock VS Razorclaw
Slag VS Headstrong
Sludge VS Tantrum
Swoop VS Divebomb
Snarl VS Rampage Omega Supreme VS Predaking
Omega Supreme VS Menasor
Omega Supreme VS Bruticus
This could be interesting. Omega kicked Devastator's exhaust so it would be very interesting to see how he would do against a newer combiner.
Devastator does appear to be the weakest combiner (Bruticus defeated Devastator in 'Starscreams brigade') but the fight with Omega in 'The Secret of Omega Supreme' did appear one-sided and he is a former guardian robot.
Defensor VS Bruticus
Haven't seen any real battle between them if I remember correctly. Bruticus got blasted into pieces at the beginning of 'BOT' and at the end of the episode Bruticus took down Defensor with that big gun. Edit: Defensor did beat Bruticus in direct combat in 'The Burden Hardest to bear'.
Defensor VS Menasor
We've had Superion fight Bruticus so why not let Defensor fight Menasor?
Superion VS Predaking
I am not sure who would win here. Superion beat Menasor and Bruticus but it is unclear how Predaking compares to these two.
Sky Lynx VS Abominus Computron VS Predaking Broadside VS Predaking.
In the cartoon Broadside is big. Which makes sense since he does transform into an aircraft carrier...

He is being pushed back by Devastator and Predaking in 'The Burden hardest to bear'. Strangely they withdraw shortly after Broadside saves a fishing boat. A full battle with Predaking could be interesting.