Don't take this the wrong way VP_commo_driver ... but ...
I am an employer, and see a lot of people cry out favouritism where I work, and quite frankly it pisses me off.... We try our hardest to not look like we are playing favourites, and even go out of our way to make sure we are not doing anything that could be perceived as favouritism. Very frustrating !!!
I know with my management style, 2 of the most important things I look at are punctuality, and reliability. If a team member can not demonstrate those 2 key trait's then all others will get more of my time to help them develop. I have some people here that I hire, and within 6 months of employment they are performing better then some of the team members that have been here from 1-5 years. I have helped assist those team members and moved them on, and the older tenured people scream out "favouritism".
At the end of the day, most companies are "Pay for performance" as oppose to the old Public Servant jobs where it was "Pay for Tenure".
I am not saying you are not performing, but have you tried to speak with your boss to find out (constructively and pro-actively) what you may need to do to show the skills that these others may have... I have monthly 1 on 1 coaching and feedback sessions where I discuss performance of the individuals, and let them know how their results are comparing to the team average. And talk about any issues I may have, or the employee may have that they wish to discuss. It is either a formal or informal discussion, and the team member sets the tone and pace of the 1 on 1.
If addressing the issue with your manager does not work, then yes, maybe it is a favouritism issue... I do know that in my years of work I have seen a pretty small percentage of people only promoted because the get along with the boss. But these people end up falling flat on their arse because the people under them can't perform. I have seen the managers "moved on" and the employees then follow the same process when a new manager takes over.
I was always taught the golden rule to management is to promote people that will be able to take your place. And the easiest way to keep your boss happy, is to keep his boss off his back ...
But anyway, my rant is over... I hope it all improves for you, but as the others have said... If you don't think the job is working, then just walk and let the fall down.