My opinion:
Fantasia is in complete command of the stage and of the audience - she certainly is the most comfortable and sure of herself. Her voice sort of bugs me at times, and I am really annoyed with the way she cocks her head to the side and sort of baby talks out a "Thank you, [Paula/Simon/Randy]" when she is complimented, but I think she is far and away the most all around talented of the group. I think also as Paula pointed out, she is good choosing material that suits her unusual voice.
La Toya, on the other hand, has sort of a default pretty voice. She's ummm, overally precise or something. Unlike Fantasia, I don't think she really thinks about what type of material she should or shouldn't sing. I have agreed with Randy throughout on La Toya - good, but nothing earth shattering.
Jasmine - such a sweet girl, I love her like I loved George, but even Jas seems to know that this is likely the end of her road. Pitch was a problem throughout the last few episodes, she is out of her league at this point but she gave it a good run.
Diana does a great job with difficult songs, particularly those that have high and low notes. I also agree with the judges in that she gets better every week.
At this point in order of best to worst, I'd say its Fantasia, Diana, La Toya, then Jasmine. They are all good though, and while I haven't enjoyed watching this season like I did last (where in hindsight I think Kim Locke was my favorite of the competition), I've still enjoyed it.

Its one of the few shows I watch religiously and I probably will continue to do so as long as its on TV.