He never actually dies

I believe that it is almost a tie between Lockdown and Megatron. The Fallen would most definitely be killed off first; we all saw how easily Optimus defeated him, though he was Powered-up. Three enemies coming at him at once would take him out. Sentinel would go next, because I hate him and he is a punk-ass.
Megatron is larger and stronger than Lockdown, so he'd obviously be able to either pound Lockdown into a pulp or simply crush him, if Lockdown weren't small, faster, and more clever.
While Megs has brawn, Lockdown has brains. And technology. Lockdown could hold his own but he would run out of tech eventually. Once out of his precious weapons all he would have is his speed and skill with hand-held weapons. However, one wrong move and he's crushed. Megatron's unbridled fury has the potential to bring even the most prestigious bounty hunters down. Plus, Megatron just never dies. He takes so many hard hits and he never goes down; lust and tenacity make for a powerful opponent. Out of all the Transformers Megatron is one of the most passionate about whatever belief he chooses to follow. His doggedness outweighs Lockdown's wile (plus, Megs can be quite wily himself.)
Sorry it's kind of messy. My thoughts were jumbled, but as you can tell, I think Megatron would win.

Although I Voted For Megatron I Think That The Fallen Is Really Underrated And That He Would Have Come Really Close To Win.Yes,I Know He Was Beaten Really Fast By Optimus Prime But By A Powered Up Optimus Prime Ugraded With Parts From Jetfire A Seeker From The Age Of The Fallen And The Most Powerful One.They Just Didn't Gave Him Enough Time And He Had Just Recovered When He Appeared On Egypt.He Was One Of The ORIGINAL 7 Primes And Most Probably The Strongest Of Them Since They Couldn't Stop Him.He Could Teleport Himself Between Dimensions And Realities At Will And Also Use Telekinisis On Every Physical Object Around Him.Finally He Could Fire Beams Hotter Than The Sun That Could Reduce To Ashes Everything They Touched.Still,The Reason I Choosed Megatron Is Based On The Decepticon Ending From The Game !!!!

Megatron is kinda OP.

The Fallen would be killed by Sentinel Prime because only a Prime can destroy the Fallen. Sentinel would probably killed in a temporary tag team move by Megatron and Lockdown since Sentinel is too powerful for one Transformer to fight alone. Then it'd come down to Megatron Vs Lockdown. Megatron has brawn, Lockdown has brains. But while Lockdown has adopted the mode of a car and can't really fight from the air, Megatron could blast him to bits from the sky in his Cybertronian jet mode.