Uniqueness: None of the SRGMP or EMAET singers had a perfectly unique voice, a voice which we haven't heard, but many singers came close to having voices which didn't resemble any particular singer and it took shape of the singer they sang.
Hemu and Sharib came somewhere close, Vishwaas on days he sings well comes close. In girls, Ujjaini was the only one whose voice is hard to classify and its a good voice. Nihira sounds a lot like Alka ji type but is versatile because she can sing an Asha Tai song but still when she sings songs by Sunidhi it loses the beauty. Himani again is super with Sunidhi songs but will have problem with Shreya songs.. Ujjaini falls in Gayatri Iyer and Mahalaxmi category, who can handle both types which is great. Hrishikesh also has a unique voice and he maintains his original voice when he sings.
Twinkle and Rajeev have 'different' voice, which aren't conventional voices and I think with good training they might come of some use.
Freshness: I felt that the younger the singer voice,the fresher their voice sounded. Thats the reason in top 5 of Challenge, Himani, Nihira, Hemu, Vinit sounded more fresh than Debojeet. Debo's voice was great but somewhere it was reminicent of Kumar Sanu. Vinit though sounded a lot like Sonu, there was innocence in his voice which made it sound fresh. In EMAET, Ujjaini, Aishwarya, Sanchali, Sinchan sound fresh.
Power: This is the factor that defines Tonal quality ultimately and a lot of singers in SRGMP C05 had it. Debojeets voice though is soothing, it was powerful. Vinit ofcousre showed us that power in Tera Jadoo Chal Gaya and the like. Hemu's nahin saamne was a clear proof of his power. Himani's Chittiye and Nihira's Kehna Hi Kya, Ujjaini's Black performance and Sharib's Aashiq Banaya Aapne clearly showed the power in their voices.
Hrishikesh and Prajakta have good power in thier voice esp. Hrishikesh.. his throw is superb. Sinchan and Irfan have good power too in their voices when they give a good performance and so does Saptak and Vishwaas.
So if it is to evaluate on all three factors, I'd have to say: Hemu, Sharib, Ujjaini, Hrishikesh, Nihira (in no particular order)