I think about any of the Summers offspring would be a formidable foe for the Dark Phoenix especially since it's said that their abilities rival her in power. ( Cable " current ", X-Man, Stryfe, Rachel. )
__________________ "I want to save the world and I have the power to do it." -- Cable.
That was Phoenix, but Galactus was exhausted (energy depleted), but in a sense he didn't lose either
When did Megatron ever have control of a blackhole? And, the Matrix has been able to take down Unicron and that posession virus (can't remember the correct name), but I doubt would have an effect on Dark Phoenix.
X-Man is genetically altered. He may burn out before he can throw in a finishing blow and I don't think any of the other offspring would do any good. Rachel's father was never truly specified. Granted, it's most likely Cyclops as I think she acknowledges him as so, but it could possibly be another's (hint hint...).
If Living Tribunal doesn't oppose her, at the very least a band of lesser cosmic beings would dispense her with little problems.
Burn out? Maybe. He's done it before when he faced Cable and when he lost his telepathy so I don't dispute the fact that though he might stand to to toe with the Dark Phoenix for awhile but ultimately she'd win out over him. But Cable on the other hand has currently displayed a level of power and control that would sufficiently rival those of Dark Phoenix. It's been mentioned time and again that he's perhaps the greatest of the mutant psionics. So I couldn't count him out when facing this threat. Unlike Nate Grey he has extreme control over his powers and has performed God-Like capabilities over the recent months. ( See CABLE #100-107, Soldier X #1-12, Cable & Deadpool #1-9 for details. )
It's due to the lack of character continuity in the Marvel Universe. One writer doesn't always observe the power of abilities of a character as much as another might. For Example in The Adventure of Cyclops & Phoenix issue #4 page 16, Rachel from the Future a.k.a Mother Askani tells Nathan ( Cable ) " Here and now, Nate, you possess as much, if not more power than me or any other telepath combined. Telepathically you are strong enough to sense a stray thought a continent away. Telekinetically, you could extinguish a star with something less than a conscious effort. " Also in " From the Books of the Askani -- X-Men : Phoenix #1-3 " Rachel who possesses the Phoenix force realizes that even its powers are dwarfed by the chosen one ( Cable ) who she awaits. Because apparently only Cable has the power to destroy Apocalypse -- which he ends up doing for the moment. And since according to Cable #29 Nate Grey's powers equal those of the Dark Phoenix and Cable/X-Man have the same unfettered power then it's apparent that Cable has the power to go toe to toe with the Phoenix.
It all depends on the writers really. They don't usually go back to other peoples work to find out how they portrayed a characters power and abilities. They do it from their own perspective.
__________________ "I want to save the world and I have the power to do it." -- Cable.
Megatron's always been able to produce black holes. You know the big gun on his arm? Yeah, its powered by creating a little warp-gate deal to a blackhole somewhere in the universe. And after a bit of self-reprogramming, megatron has taught himself how to use that power to create blackholes wherever he wants.
and the matrix... well... it is suppsed to "light our darkest hour", and seeing as this is DARK pheonix we are talking about, and it is a threat to all life (just as unicron was), im willing to bed that "an autobot shall rise from our ranks... and use the power of the matrix... to light our darkest hour..." know what that means? Bye-bye birdie.
This is how it happens! Somewhere on the outerlimits of Genosha...
Rougue: Man she is really annoying.
Cyclops: (whimper) Why?
Wolverine: Because you touch yourself at night!
Cyclops: I have a clear shot...
Prof X: No Scott, you suck to much!
Jubilee: You realize our universe is at stake right?
Prof X: Not now Jubilee, if I can focus I may...augh!!!
Cyclops: Whats wrong?
Prof X: I...need...(cough) heelp!!!
Wolverine: Well...that didnt work!
Cyclopswhimper) I will find you Jean!!!
(The group stare at Scott horrified)
Jubilee: Does this affect my grade?
Cyclops: Oh Jean...I should of had sex with you when I had the chance!
Wolverine: (mumbling) damn pussy
Rogue: Its ok lil' buddy (goes to pat Scott)
Prof X: Jean...come back! I am in your mind.
Wolverine: Great! Now the bald man is acting up again.
Cycops: What? This is a great man.
Jubilee: Yeah! Show dick sum respect.
Prof X: Jean...why must you continue this? augh!!!
(Proffessors head begins spinning until it pops off)
(Darth Vader enters)
Vader: Well...this is indeed a distrbing universe.
(Everyone runs away)
And yes, Starwars was a comic series!