Quercus Alba was the most Evil (And most hardest to defeat) suspects I've dealt with in all of my time playing Ace Attorney.

Both of them were masterminds behind their own crimes. Morgan was behind the murder of Dr. Grey and technically, the true boss behind Elise's/Misty's death, because without Mommy's intervention (a.k.a. scheme), nobody would have channeled Dahlia. Obviously Dahlia couldn't have done anything to Elise on her own; she was already executed and therefore, dead. However, Dahlia was in charge of the Dusky Bridge incident, successfully manipulating Valerie, Iris and Terry. She also planned to kill Doug Swallow and eventually, Phoenix without anyone else's help. Now, in those two events, Morgan had no direct involvement.

However, let's think back to their past. When Misty usurped the title of Master from Morgan, demoting the latter to the matriarch of the branch family, that was when Morgan decided that she wanted what was rightfully hers back. "Rightfully", because we can't blame Misty for being more adept at spirit channeling despite being the younger sister. So unfortunately, Dahlia and Iris were caught up in Morgan's plot to regain control of the Fey family. One might say that this was one of the catalysts that led to Dahlia becoming a criminal at the young age of 14, and led to Dusky Bridge, Mia's second case, and even to the Bridge to the Turnabout.

So in that sense, you could say that even though Dahlia is "evil" (I use quotation marks because everyone's definition of evil is relative), Morgan had a hand in making it so. Not only that, she continued on with her own plans to get what she wanted, even if it meant manipulating and even killing anyone she had to.

Motives. Morgan wanted fame and power - if not for herself, for Pearl, because that would automatically translate to fame and power for herself as well, being the mother of a young channeling prodigy who would someday control the Fey family. I can see the future - Pearl would be reduced to a puppet, and Morgan would be pulling the strings, the true power in the clan. And of course, Morgan held a grudge against Misty for being...well, better than she was. Revenge. Greed. Ambition. Pride. And Morgan was willing to go to great lengths to achieve her goals any way she could.

As for Dahlia, her mother didn't want her and her father was terrible at parenting, let's put it at that. So she started plotting against Mr. Hawthorne, bringing Iris in because...well, Iris loved her sister and was willing to help her in any way she could. Don't forget Terry and Valerie, lured in by the promise of great wealth. Doug was killed because he saw right through Dahlia. And Mia was the lawyer responsible for finally putting an end to her life of crime - or so they thought. At first it was for greed and revenge, and then it was simply a vendetta against the attorney who had stopped her. Dahlia went so far as to target Maya Fey next - because the latter was Mia's little sister.

Thus, Morgan and Dahlia set their sights on a common enemy for different motives, but were both responsible for nearly ruining Maya's life - the life of the next Master, Misty's daughter, and Mia's sister.

And because I'm hungry, I'll wrap this up. Dahlia may have killed more people along the way and would have killed more had she not been stopped, but it was just that Morgan was more sophisticated in the scheming department, and so she had less need to murder anyone who got in her way. Then again, technically the only people "in her way" would be the main branch of the Fey family. So body count alone isn't an accurate measure of how evil anyone can be. We have to take into consideration the circumstances, as well as the reasons and the histories of these people. While Dahlia was affected by Morgan's early schemes, things would have turned out a lot better had Morgan submitted to Misty and left well enough alone.

Although I'd say Morgan has the edge for starting it all. If she had only been a better mother and a better person, Dahlia probably wouldn't have grown up that way. And Iris? See, that's where the nature part comes in - personality. Iris COULD have gone down the same path as Dahlia if she had been a slightly more different person. Like...more like her sister.

I'd say Dahlia,though Morgan is half responsable for the way Dahlia became. Morgan wanted kids only if they had high spiritual power.The Dad wanted to be powerfull,that's the only reason why he married Morgan anyways.So once they figured out that The Twins wern't powerful,Morgan chucked them and moved onto more plotting. Leaving Dahlia and Iris to Mr.Hawthorne was probably what started this Devil Dahlia to emerge. With Iris,the only one she could turn to and cared for her,gone because her new mom only wanted 2 damn kids,Dahlia started hating her Father more and more,and she wanted to make sure he knew it. She did that by...
1.Plotting a "kidnapping" with Valerie and Terry
2.Stealing a 2 MILLION DOLLAR Diamond and running away with it
3.Coming back five years later to kill Valerie,and unintentionally Terry.
But it didn't end there,for she Tramatized the young new Defense Attorney that handeld the Fawles case Mia Fey (her soon to be nemesis) thus enraging Mia's Mentor and soon after Boyfriend Diego Armando.The two swore to get Dahlia convicted,which arose problems for Ms.Hawthorne. BIG problems. 6 months past as Mia and Diego fell more in love and more into Dahlia's dark secrets. So finally Dahlia figured she had to shut up Mr. Armando for good by poisonning him. I guess she figured she hit two birds with one stone since it hurt Mia extreamely. But she made another horrible mistake by poisoning him that day.
Moving on...
She had to get rid of any evidence of the poisoning,so she picked a random young fellow out of the crowd named Phoenix Wright to be her little clueless helper.She gave him the necklace that held the poison as a "love gift" and made a run for it.
8 months (i think) after that she finnaly decides to kill phoenix but instead kills doug swallow,as proven by miss Fey. She was put in jail and soon killed.
Thus it was the end...or was it really? She did yet ANOTHER crime at Dusky Bridge 5 years later,even though she was already dead. And what was her goal in that crime? Oh well just getting more revenge on her DEAD NEMESIS MIA FEY.