It's a hard question, 'cause all the SHs are essential. All of them "saved the day" at least once or have an essential role in the crew (Nami and Robin particularly). A lot of people here is saying that Usopp is trash, but he possibly is the one who saved the crew the most. Here are each one's feats:
Luffy
- Captain, beats the arc's main antagonist (almost) everytime;
- Saved every crewmember at least once (except Jinbe);
- Joy Boy 2.0.
Zoro
- Saved Luffy, Nami, Robin and other crewmembers several times;
- Beats the second strongest enemy and at times hold his own against the main antagonist;
- Faced Kuma to save the whole crew back in Thriller Back;
- Took Kaido and Big Mom's Hakai head on and still helped the Alliance by defeating King.
Nami
- Navigator, probably the best in the world, essential to get to the end of the Grand Line;
- Saved Luffy, Sanji, Usopp a few times;
- Usually beats an enemy stronger than herself.
Usopp
- Was the only one capable of defeating Perona, thus saving the whole crew in Thriller Bark;
- Saved Luffy and Law from Sugar, thus saving the day in Dressrosa;
- Took care of Going Merry until the end, and the ship ended saving the crew in Enies Lobby;
- Motivated Luffy to get up and finish Rob Lucci in Enies Lobby;
- Powered Nami up with the clima tact.
Sanji
- Saved Luffy, Nami, Usopp, et al. several times;
- Beats the third strongest enemy and sometimes hold his own against the second or even the main antagonist;
- Opened the Gates of Justice for the crew to escape Enies Lobby;
- Incapacitated Big Mom with the Wedding Cake.
Chopper
- Doctor, essential for maintaning the crew's health against the Grand Line's dangers;
- Inadvertently saved Nami in Enies Lobby, by breaking the bathtub and washing Kalifa's soap;
- Saved Sunny and the crew from Caribou's ship near Fishman Island.
Robin
- Archaeologist, essential to deciphering the poneglyphs and thus reaching Laugh Tale and the One Piece;
- Saved Nami, Franky, Usopp, Law and others a few times with her DF powers;
- Saved Sanji from Black Maria and ended up beating her.
Franky
- Shipwright, responsible for the construction of the Thousand Sunny and its maintenance;
- Sunny's Coup de Burst saved the crew a few times;
- Provided a distraction for Usopp (and Robin) knock Sugar out and liberate the toys in Dressrosa;
- Protected Robin a few times (Enies Lobby, Thriller Bark, etc.) and Sunny too (Punk Hazard).
Brook
- Took a copy of Big Mom's Road Poneglyph in Whole Cake Island;
- Saved Zoro and Usopp from drowning in a cage with his powers in Fishman Island;
- Saved Franky and Robin from Tararan (the spider zombie) in Thriller Bark.
Jinbe
- Helmsman, saved the whole crew in WCI arc;
- Helped Luffy escape Impel Down and saved him from Akainu in Marineford;
- Saved Luffy again in WCI, by releasing him from prison;
- Gave his blood to Luffy after his battle against Hody Jones.
Considering the above, I'd say that Brook is the "least essential" strawhat, but without him the crew wouldn't be where they are now in the One Piece race and some of them (including key members like Nico Robin) would be dead.