I feel that Rina is the most talented of the band. Second for me is Mami, and third is Tomomi. Here's why:
Rina: she can play drums, she can play piano ( a very hard instrument), she can play guitar (Koshi-Tantan solo, anyone? Note: studio version only), she obviously has an understanding of the bass, and guitar, because send composes, she can sing and dance, and she can write. What else do you need? It's amazing to be able to do all these things. And she can play the shaker and tambourine. And she is a determined musician who is always working to make her work better, and she has an unbelievable consistency in her live playing.
Mami: she plays guitar her great( her solos are crazy sometimes IMO, and she plays them very well. I'm not a guitarist, but I am a bassist, and I can see if something is difficult, and some solos look very hard. Again, my opinion.), she plays drums well, she draws the best of the band, she cooks the best of the band, she is a gamer, she can sing backup well and lead vocals, and she makes some great compositions(Want You, and Hikare with Rina), and her lyrics aren't too shabby( Queens are Trumps, Want You, Shoujo M). She may not be too consistent, but she improves, and she tries her best.
Tomomi: an amazing bassist. A great singer( although lately it has been iffy). She writes touching lyrics, and I don't know if she composes or not. I know how hard it is too sing and play bass. I've tried while playing bass(like Shoujo S, and Koi no Kajitsu are hard. Kagerou is pretty easy). Her backup vocals sure good too.
Haruna is a great singer and dancer, but I feel like her guitar playing skills aren't lived up to. She can play without looking at her guitar, and that's amazing.
But SCANDAL is a talented, determined band so they're all special.