Lucia hands down.
DMC2 is the latest in the DMC time line, when Dante is considered to be at his strongest. Dante and Lucia tend to fight against many of the same bosses, and what Lucia lacks in power, she makes up for in speed and combo capabilities. She can take down bosses just as quickly as Dante can, if only because of her speed and the amount of hits she can get in over a short period of time.
Also, take account Lucia's Arial special: by pressing the slash and shoot button at the same time, while air born (or under water) in Devil Trigger, Lucia is capable of sending orbs of light into the sky and have orbs of light energy rain down on enemies. It takes a good chunk of her DT gauge, but deals a TON of damage, about the same amount of damage as Majin Form Dante's abilities. And she doesn't even need her health to be in the red to do this.
Really the only thing that makes Trish an effective fighter, from what I've seen, is when she has access to the Sparda sword. Without it, she doesn't seem to be all that powerful. Yes as Gloria we see her take down demons fairly easily with her dagger, not needing the Sparda, but those demons aren't exactly intelligent. In a fight against Lucia, a skilled fighter that specializes in the use of throwing blades, Lucia could probably deflect the dagger easily.
Also just like Trish, Lucia is attributed to an elemental ability: Ice. She starts out with the Ice Heart, which imbues her attacks with ice elemental during DT. And her DT art works depict her swords glowing blue, which only happens when the ice heart is equipped. Considering her art works portray her with blue eyes, and in game with blue eyes regardless of the elemental heart, its safe to say that ice magic is as innate to Lucia as Lightning magic is to Trish. Lucia could, theoretically, simply create ice pillars to act as lightning rods against Trish's electric capabilities. As well as any possible form of ice magic, ie creating it from the moisture in the air, and controlling it with her demonic powers in any way she wants.
Really, remove Trish's use of the sparda, and find a way to disable her electric magic, and Trish isn't all that effective in battle. Not to mention her young age and inexperience in battle: she was created for the sole purpose of tricking Dante so Mundus could kill him, she was never made for battle and had to start learning to fight effectively after the events of DMC. Where as Lucia was raised among a clan of Half Human, Half Demon, Devil Hunters. Shes been taught in the art of combat since early childhood, or so we are led to believe. So even if she is somehow disarmed, there is a good chance shes been trained in hand-to-hand combat as well. After all in DMC2 a good number of her attack combos involve her performing powerful kicking strikes.
Really out of all the females in the DMC universe, Lucia is probably the absolute strongest. She may be an artificial demon, but her powers are supposedly on par with Dante's, and trained in combat since early childhood. Trish is a pure demon yes, but she was not made to fight and had to learn to fight. And Lady, while a skilled Demon Hunter, is still only human.