Parkinson, whose side are currently sixth in the table, three points behind Cheltenham who occupy the third automatic promotion spot, said there was little to choose between any of the play-off contenders.
He said: "Gillingham and Port Vale are the obvious stand-out teams for me. We played well against Port Vale, but I saw them play against Oxford at home and they were excellent that night and Gillingham on the first day of the season were strong.
"They've had a little bit of a wobble, but they've got a good side and a strong squad. Those two and Fleetwood and Rotherham and teams like that, it's so competitive this year, really competitive."
Meanwhile, Parkinson is hoping injured pair Kyel Reid and Zavon Hines will both be back in contention for the Christmas programme.
Winger Reid has missed the last 14 games due to a groin injury and Hines has been sidelined since the middle of last month with an ankle problem.
Parkinson added: "Kyle and Zavon have been on the pitch (on Monday) down at the ground with the physios doing a session and I'll know a little bit more once I know how they got on.
"But they're progressing well. We've got to be looking around the Christmas fixtures. We're expecting they're both going to be in contention then."