Tirek could just absorb the power of any villian that fought him. But as far as raw power is concerned, I dunno.
Even at full power (minus Discord's magic) I bet Discord could beat him in a straight fight. He effortlessly warped all of ponyville upside-down. If he wanted to I bet he could overpower just about anything. Heck, if he could outsmart Tirek, he could probably even beat Tirek when using his own chaos magic against him since Tirek would probably be more inclined to use normal magic like in his fight with Twilight.
Then again, maybe he couldn't absorb Sombra's magic? After all, he's an entity of shadows, how do you contain an entity like that long enough to steal its magic?
And the Dazzlings might have a chance if they could get a song in before the fight. Depends