First, you're comparing apples to oranges here. The Celebrity Equinox is far superior to any of the others on your list.
In my opinion after doing this for more than 10 years, having over 2500 clients, and doing 43 cruises myself, I would personally rate them Celebrity as a 4-star cruise line and all the rest on your list as 2-star.
The NCL Epic is the only ship currently offering cabins specifically for singles.
Keep one most important thing in mind - the advertised price consists of two fares; the cruise fare and the port fees. Normally, when a single travels, the cruise line will double the cruise fare (called 200% single supplement). Also, some cruise lines will wrongly charge double the port fees and others will do it properly and only charge single port fees. NCL is one that charges double except when the single cabins on the Epic are booked. So make sure you look at the bottom price when considering these cruises and not just the advertised price.
But as for which of these is better for a single female cruiser, it really depends on what you're looking for, your lifestyle, your age, and your budget.
For example, Celebrity will draw an older crowd, but the food is definitely MUCH better than any of the others on your list. Costa is alot like Carnival, but with a heavy Italian flare. Many Americans aren't big fans of MSC as it's also heavily European.
Best to talk to your agent so they can advise you on the differences so you can make an informed decision. Remember, what one person loves, another will hate, so stay away from recommendations. It's best to do your research, read reviews everywhere you can find them, and then discuss with an experienced Cruise Specialist who personally knows all of these cruise lines.
Pete