Okay, here's how I see it going down.
Yes, Killian is probably stronger, has his heat-related powers and regeneration - so that's a couple of physical advantages.
Drax, well probably not as strong, but extremely tough. ( he was strong enough to fight and kill Korath, who withstood blasts from Star Lord's guns without any lasting injury).
However, Drax has a massive psychological advantage over Killian - he's absolutely crazy, clearly has no fear of death and doesn't hesitate to kill an opponent (in unusual and gruesome ways if necessary).
While Killian is a powerhouse, underneath all that he's still just a bitter nerd - Drax has a heart of gold....but he's also total lunatic, willing to jump inside a huge monster and try and cut his way out and rip out part of a cyborg's brain with his bare hands.
I see the fight going a little like this.....
Killian beats Drax savagely and burns him with his heat powers (although we know from GOTG that he's reasonably heat resistant). Drax cuts him a bunch of times, which heal quickly. Drax then realises that he'll have to take off something that Killian can't grow back and cuts off Killian's head or reaches down his throat grabs his heart and pulls it out.
Also, Drax has something else that Killian doesn't - family. If Killian ever got the upper hand and it looked like he was going to kill Drax, then the rest of the GOTG would step in and take Killian down - maybe they'd drop a planet on him, or Rocket would build some giant space cannon to disintigrate him (although Groot would be in big trouble against Killian).
Having said that, power alone isn't everything - while Rocket Raccoon may lack the physical power to take on Killian directly, no doubt he'd whip up some amazing gadget to neutralize Killian's powers and take him down - Rocket is a crafty little bugger and (like Drax) not afraid to kill an opponent (he's much more ruthless than Tony Stark).
As far as your power tiers go, personally I would rank MCU Scarlet Witch a lot higher, at least in the next tier.
Her psychic powers can affect physical things and can be used both offensively and defensively (and for limited levitation/flight) but she can also mess with an opponent's minds - I mean, in Age of Ultron she pretty much takes out Thor, Cap, Black Widow and Hulk single handed. MCU Vision is much more poweful than his comic book counterpart, but she takes him down with relative ease - she takes control of his powers and uses them against him (and he's got an infinity stone on his forehead).
In some ways Wanda is the most powerful Avenger, because nobody seems to be able to defend themselves against her mind-messing powers (except Hawkeye). Probably the one MCU character who could resist her powers would be Doctor Strange - and yeah, Dr Strange with the Eye of Agomotto pretty much beats anyone, I mean who could beat a guy who can create time loops ?