Boyz II Men had/have three strong lead singers, the harmonies, the songs and everything about them came together perfectly. I'm not just saying this because the were one of the most successful R&B group of the 90's but their vocal chops together and individually were that great. En Vogue were incredible in their own right and you can definitely make a case for them as well because vocally they were incredible with individual parts and harmonies too, but I'd say I prefer Boyz II Men. That's not to discredit Jodeci, Dru Hill, Silk, TTT, Shai, LSG, etc. Great question.
I would say Jodeci, although I'm only saying it because I'm a self-confessed Jodeci "mark" lol. Although I do genuinely think K-ci and JoJo had better voices overall. I just think its a tragedy they all got into drugs and booze, really spoilt the group. Devante could have been a musical powerhouse producer today if he never let the coke get the better of him.