My most memorable and touching foster experience would be Marcus.
The rescue had these pictures posted on their Facebook of a sad pittie with tubes coming out of him everywhere, asking for someone to foster him until he healed (he was either used in fighting or just in a horrible fight). We had another foster already, but I figured he would just be relaxing in his crate while he was here. Just days after surgery he was bouncing around in his crate, barking and smacking his cone up against the sides. He drove us crazy! Lol. But we fell in love with his energy and his love for life, which is why we decided to keep him until he was adopted. We were really hesitant to put him with our dogs right away or correct his bad behavior because of his past, but it was like nothing had ever happened to him. He was always affectionate. Very stubborn though! In the beginning of leash work he would get mad that I was telling him what to do and he would sit down and refuse to move, haha. But we worked through all that and by the time he was adopted I really didn't want to let him go.
I still have contact with his forever family though so I get to see happy pictures all of the time. They love him so much and every time I see him happy it reminds me how amazing fostering really is and what a difference we make.