Neither David Crosby nor Graham Nash played lead guitar in CSN, only rhythm. Stephen Stills was their lead guitarist, and with no disrespect to Crosby and Nash, Stills was far and away the best guitarist of the three. In fact he's widely considered one of the best guitarists in popular music history by people who actually know music history. (Stills was not only their lead guitarist, he played every instrument except the drums on their debut album.)
When Neil Young was with the band, he played lead on some songs, but Neil himself will also tell you that Stephen is the better guitarist, so I disagree with the other answer implying that Neil is better than Stephen.
Neil also said — and I concur — “Stephen is a genius”.
Steven Stills was always the lead guitarist of CS&N. In fact, he played most of the guitar on thir first album (including all the parts on “Wooden Ships”, a truly rocking guitar piece). He is an amazing guitarist - Jimi Hendrix jammed with Stills often, and always recognized him as one of the best guitarists in rock music.
Stephen Stills played lead guitar for the group… and most of the other instruments on their first album. His nickname in the group was “Captain Many Hands”. Also, if there was one person in the group who could be considered the “lead singer”, that was also Stephen, who had the most distinctive voice of the three singers. Unfortunately, as is often the case with raspy-voiced singers, his voice has held up the least well over the years.