2. Craig Mackail-Smith
To truly judge how vital a player Mackail-Smith is, you have to see him in the flesh.
His work-rate is genuinely staggering and during his time in a Posh shirt nobody could ever accuse him of lacking committment.
Not that effort was all he gave Posh, Mackail-Smith's pace caused defences no end of problems during his time at London Road and it's fair to say he became one of the most feared strikers outside the Premier League.
His form had been excellent prior to the 2010/11 season, but it was in League One that the striker's form peaked.
Seemingly on a one-man mission to get Posh back to the Championship, a league they had been relegated from the season before, Mackail-Smith bagged 27 goals in a campaign that saw Posh score over 100 goals as a team.
They did, indeed, return to the second-division, but they had to play the next season without Mackail-Smith - who completed a £3million move to Brighton during that summer.