Every comic book character has a run from a specific writer who nails the personality of that character down to a tee, and that fans often universally name that writer the character's best. For the Punisher, fans unanimously agree to it being Garth Ennis, for Captain America fans agree it was Ed Brubaker, and for Daredevil it was Frank Miller.
For Deadpool, that writer was Joe Kelly. Who's run on Deadpool was nothing short of a masterpiece. He was the writer who shaped and defined what Deadpool was supposed to be and helped mold him into the mercenary we know and love today. Joe Kelly crafted Deadpool from a just a loud-mouthed mercenary into a disturbed man with deep psychological trauma and often had that character experience various ups and downs in his journey to become a hero and a better person. He also laid down the groundwork for what Deadpool was supposed to be, and even was the writer who first had Deadpool break the 4th-wall. The true achievement of Joe Kelly's run was that he actually had readers care and worry about a disturbed man who enjoyed torturing blind old ladies, and often bullied his "best-friend." Something not easily done.
Level of Recommended Reading Importance - 10/10 (A must read for any Deadpool fan, the definitive Deadpool run).
After Joe's run, fans will most likely agree that Cable and Deadpool was the best run to come afterwards. Teaming him up with the time traveler and former enemy and creating one of the best friendships in comics. Fabian Nicieza returned to write the character he created, and was also the writer who helped establish Ryan Reynolds as the perfect man to play Deadpool if a live action film was to ever be made. Near the end of the run, Cable left and Deadpool became the center of attention as readers watched as he went on various adventures and teamed-up with various superheros around the Marvel Universe including Doctor Strange and the Fantastic Four. The run also added a new member to Deadpool's very short list of friends by bring Bob, Agent of Hydra into the picture.
Level of Recommended Reading Importance - 8.5/10 (Great series, great humor, great friendship dynamic, but a few plot-hole to past issues).
While a controversial writer, Daniel Way is still the writer who has had the longest run on Deadpool yet with a total of 65 issues. While some older fans dislike Daniel Way for reasons such as giving him voices in his head with no explanation behind how they got there, or turning Deadpool into more of a loveable goofball than a mentally disturbed mercenary. Daniel Way's run is easily one of the most popular runs Deadpool has ever had. Introducing the character to new readers and turning Deadpool into a more popular marketable character. He's also responsible for bring Deadpool into middle of the Marvel Universe and having him play a major part in Marvel's big crossover events. It was Deadpool who gave Norman Osborn the knowledge to defeat the skrull's during the Secret Invasion, and it was Deadpool who helped the x-men destroy some of the vampires during the Curse of the Mutants story arc.
Level of Recommended Reading Importance - 5/10 (Fun series with some great ideas, but many unexplained changes to the character and the occasional plot-hole).
Often not thought of as a team-player, Deadpool finally joined a team called the Uncanny X-Force written by Rick Remender. Joining other super-powered killers such as Wolverine, Psylocke, Fantomex, and Archangel. Deadpool while often not the focus of attention was always there to lend a helping hand when needed. Rick Remender's run was a masterpiece and helped show the other heroes of the Marvel universe that while he may be an annoying mercenary out for a paycheck, he is still a force not to be messed with and a person who only wants to feel important. Deadpool was also a way to bring some lighthearted comedy to ensure the book didn't become too dark and depressing.
Level of Recommended Reading Importance - 6/10 (Amazing series, but Deadpool isn't in the spot-light for the majority of the series).
While still new, the 2013 Deadpool monthly series has entered with a bang, and is now the perfect series for readers to go to when looking for a fun, enjoyable read. While comedians Gerry Duggan and Brian Posehn have promised to explore Deadpool more as a character and have also promised a darker tone in future arcs, their first arc involving Deadpool hunting down former USA presidents on a mission to destroy the world, is loaded with laughs and sprinkled with fast paced action. Returning Deadpool to the deeply psychologically disturbed anti-hero he was, and finally giving an explanation behind the voices in his head. While it's still too early to say if the quality will remain entertaining, it's safe to assume that this series is a great starting point for anybody wanting to get into Deadpool.
Level of Recommended Reading Importance - 7/10 (Well crafted intertwining stories, return to a darker Deadpool, very humorous).
Deadpool: Wade Wilson's War
Deadpool Kills the Marvel Universe and it's sequel Deadpool Killustrated and the third in the "Killogy", Deadpool Kills Deadpool
Deadpool Suicide Kings.