Mr. Big appeared in the very first episode.  Samantha gushed to Carrie that he was poised to be the next:

Charlotte thought she found her knight-in-shining-armor in Trey, the physician from a blue-blood family.  Unfortunately, this issue was a deal-breaker for their marriage:

On her last date with Big before he left for Napa

Attending a party in honor of her book launch 

Through a message on her answering machine 

His agent at their shared publishing company told her 

He refused to cut off contact with his ex-wife 

He was going to work in Paris for several months, and didn’t invite her

Attending the opera then slow-dancing in public

Sending two dozen flower bouquets to her apartment