I can really only answer this from S5 onwards.
Tessanne (S5): Bridge Over Troubled Water - I think she was a mid-packer for a while, but this basically guaranteed her the win.
Josh (S6): All of Me - He was practically even with Christina at this point, but since she had an off-week during the semifinals this segmented Josh as the one to beat.
Craig (S7): The Old Rugged Cross - This was the only gospel song of the season and it gave Craig a boost that was practically unstoppable.
Sawyer (S8): I'm A Man of Constant Sorrow - He won the whole season during the first 5 seconds of this lol.
Jordan (S9): Chandelier - The same thing as Sawyer. As soon as he opened his mouth no one was stopping him.
Alisan (S10): Blue Bayou - This was honestly kinda hard to decide since I thought she was neck and neck with Adam (hence why I was kinda convinced to put Let Him Fly here), but considering the insanity that this audition was, it was a rather obvious choice.
Sundance (S11): Me and Jesus - No One basically guaranteed him a finale spot, but his lead wasn't big enough to be considered a victory lock. The fact that THIS hit #1 on iTunes (and this was a performance I have a strong dislike for because I found it extremely plain and irritating) basically meant Sundance was unstoppable.
Chris (S12): Rhythm Nation - He probably would have lost if he sang... well anything else (except maybe another gospel song). I thought he had a chance to win after this despite Lauren's monster of a seemingly unstoppable iTunes lead, and as soon as they said the voting was close I knew Chris took the win.
Chloe (S13): Landslide - I think this is what gave her a ton of fans that never stopped voting for her no matter how average she was.
Brynn (S14): Here Comes Goodbye - This was probably the most difficult one to decide since Brynn never really struck me as a "frontrunner", I had her pegged for 3rd place until finale night after all. That said, if something definitely put her on the radar it was this.
Chevel (S15): Little White Church - Another one that was pretty hard to decide. Travelin' Soilder was a huuuuge hit on Facebook as mentioned, but I think Little White Church is what put her in a great spot. Her support was extremely high for an uptempo song, and if that happens then ballads are more often than not just as high if not more.
Maelyn (S16): Stay - The Scientist was a breakout moment for her since it took her from "You're not beating Gyth sadly." to "COME ON BEAT GYTH INTO THE GROUND!" I put Stay on here because Gyth pandered EXTREMELY HARD for votes and yet Maelyn STILL outcharted him that weak and managed to get the streaming bonus again, which was HUGE. So huge in fact that it was basically game over. She didn't need Hallelujah to win, but it was essentially to say "Hey, I know that I'm the frontrunner and I am NOT risking losing it."
Jake (S17): Cover Me Up - He had no chance of losing after this. It gave him a fanbase that voted for him nonstop (not to mention his "stream the tar out of my song" advice for his live performances).
Todd (S18): Love, Me - Funnily enough, a guy I have described as a hardcore panderer managed to secure his win with a song that WASN'T gospel, lol. As soon as they showed his sleeping daughter on the camera it was game over.