I feel like Lucy's trump card would be Gemini/Gemini stardress. With her power upgrade she should be able to copy more mages, copy them for longer periods of time and copy stronger mages. Thus this dress should give her power and variety. By feats she should be able to copy mages like Mirajane atleast.
Mirajane magical power dries up water:
Lucy's magic power dries up water:
If we were to believe Hiro and Lucy when they said the stronger Lucy becomes so do her spirits. Then around x791 she would atleast be 6x-10x stronger than she was in x784. So Gemini should be able to copy around 12-20 different people and hold their transformation for around 30-50 minutes. Rather Hiro would have been consistent with this or not is neither here nor there but by logic Gemini should be Lucy's trump card.
Based off how I would rank them based on what powers they should posses I would say:
1. Gemini Stardress: A versatile Dress with the ability to copy others magics, appearances and minds. The dress would off her the greatest range of power.
2. Capricorn Stardress: Human Subordination is a powerful ability to allow the user to weaken their opponents power and even enslave them. Even if faced with a non human the merge technique could prove very useful if say facing off against a powerful Beast. Although people go back and forth as to rather this was solely Zoldeo magic or both I lend towards both. Since Leo know about Capricorn possessing Human Subordination before he even found out Zoldeo was possessing Capricorn. This means Capricorn had to have had dealings with this magic before he came into Zoldeo's hands as that was the last time Leo saw him.
3. Aquarius Stardress: Has the ability to manipulate huge amounts of water and even manipulate huge chunks of earth turning them into a liquid form. Also has a powerful water barrier able to block powerful nukes.
4. Sagittarius: Has a high attack power among all the Star dresses. Long range combat and can target multiple targets at once.
5. Scorpio Star Dress: Has the ability to manipulate huge quantities of sand and offers Lucy limited flight.
6. Taurus Star dress: An immensely physically strong Stardress. Would be great for CQC.
7. Cancer Stardress: Deadly scissors make for a great close combat dress.
8. Virgo Stardress: Great for stealth and speed. Offers her the ability to trap and attack her opponents from underneath.
9. Leo Stardress: Among all her CQC dresses this does off her the highest attack power with Regulas but the design and her lack of training greatly hinders the effectiveness of the dress.
10. Aries Stardress: Whilst a beautiful form it lacks any real offensive abilities. Although good for certain defensive power.
If we include the possibility of Silver key Stardress than Horologium would be 11 given it's non offensive abilities but extremely high defensive capabilities. Crux would be number 1 givens it's vast knowledge and ability to download info. Could easily allow Lucy access to uber Celestial spells and different types of magic used by the Spirits themselves such as Copy magic, water magic, light magic, etc.
