The IMDB rating is highly subjective. The arithmetic used for averages or exclusion of voters for the top 250 are a way to adjust the ratings to misuse or exceptional values. But they work on the elementary data point which is bluntly: How many "stars" do you give this movie?
This is not an objective data point in any way. It isn't even explained what this value should express. Is the user expected to rate the movie quality, as "objective" as possible for him? Or should a express if he liked the movie? Overall or will certain special features influence the overall rating (e.g. the presence of a favourite actor or a brilliant soundtrack in an otherwise "bad" movie)? There are movies I love but I know they are of low quality in general. I like them because of a certain song in the soundtrack or a special memory they raise. For me those are ten-star movies. By any quality measures or how I would estimate them to be liked by others some of them are at best mediocre. Other movies I have to admit are technical and narrative brilliant, influenced the movie world - and bore me to hell. What rating should it get from me? I seriously don't know.
The basic question is not only highly subjective but also unclear. Without explanation the user enters a value of one to ten "stars". It is completely up to the user what this means to him. Without any doubt your question about objectivity can be answered only with no, it is not objective at all.
But you ask for credibility at all and better ways of rating.
IMDB is the most famous movie site (by my estimation). Even lesser known movies get some attention there. But as always most attention is by fans. Popular movies with a high fan base get much more attention than others, and mostly positive. Just look at the "top" list (or the "bottom" list for the same reasons). It seriously differs from "best of" lists of established institutions, critics, or media. The proportion of fandom movies (Lord of the Rings, Star Wars, Pulp Fiction, Matrix...) at the top is definitely higher. This doesn't mean one is "right" or "wrong" but the difference is apparent.
The alternative are "qualified" reviews, e.g. averaged critics by professionals who tend to be more unbiased against fandom. This is what sites like Metacritic or Rotten Tomatoes do. Panels like the Oscar committee or are also an alternative.
The problem here is in selecting the critics who participate. If you select a small homogeneous group it will be biased to certain movies. Select a panel of Swedish intellectuals to decide over the "best" (by certain criteria) literature in the world in the last year and you will get a high proportion of Swedish literature. Their expertise with e.g. Chinese literature may be not so high.
I think that sites like Rotten Tomatoes or Metacritic do a good job of averaging it out. You can get a "popular" vote aside of the critics vote to see how they differ; and they often do. A successful movie doesn't have to be a technical or narrative good one. That doesn't make it less enjoyable but influences the typical "popular" vote. And that is what IMDB delivers.
So what rating system is "best" depends highly on what is "best" for you. That is a little bit vicious, of course. I personally think that you should find and follow a few bloggers, professional or not, that you like and that typically hit your taste. That is highly subjective but after all enjoying a movie is highly subjective. Apart from a very rough estimation I don't see much value at all in a single aggregated rating value without explanation of its consistence. Discussing why one movie is a 7 and another an 8 is beyond my comprehension.