Yui - too frilly and pink, looks like something a five year old puts on a barbie
Mugi - too plain, no personality
Mio - impractical for beach activities, looks like it'll fall off if she does anything other than tan
Ritsu - multiple colors; sleek, aesthetically pleasing design; strikes a balance between showing the goods and functioning in high-fun environments
Conclusion - Ritsu has the best taste in swimsuits and is the optimal choice for accompaniment during summer activities.