Strictly speaking from a playback singer perspective and ignoring "cuteness" and body movements/posture, I think Sreeram is the complete package. He has such a strong playback voice, excellent expressions, and great control over his voice. I can easily close my eyes and picture his voice on any current Bollywood actor.
I love Shivam, but I feel that he somewhat lacks in his voice quality. I don't hear it and automatically think wow! we need that in Bollywood today. He has stage presence and the X-factor, but I think strictly speaking of singing, he is weak as compared to Sreeram. Also, Shivam is young, and that could be very well be the reason.
With Rakesh, I feel he lacks versatility. He repeats the same expressions in all his performances, which makes his versatility limited. Rakesh is a good singer, yes, but I also feel that Rakesh only sings certain types of songs well. If he steps outside those boundaries, I think some of his weaknesses will become more obvious.