If you're arguing about the best #25 in Falcons history, it's a short debate with only one answer. Save yourself the time and embrace Scott Case.
Case is simply one of the best Falcons defensive backs ever. He was known for his physicality and aggression, and during his eleven seasons in Atlanta, he put up a combined 30 interceptions, 7.5 sacks and gobs of highlight reel hits. The Falcons' defense was hit or miss throughout much of his tenure, but he was a key player on some very good units, and is a legend for good reason.
His best season was probably 1988, when he went to the Pro Bowl after grabbing an eye-popping ten interceptions, or a full one-third of his career total.
An honorable mention goes to William Moore, another insanely talented, hard-hitting safety who has been a true asset for the Falcons over the last several years. His injury history has held him back more than anything, and it's always possible that if he hangs around for a few seasons more, he'll surpass Case. There's nobody else on the list that really approaches those two guys.
A hearty salute to Case.