My rating system is very simple. The possible scores are 1-5, with a single special category, 1*. Note that these categories cast a wide net, so 2 things with the same score may not be of the same quality. I make no attempt to be objective, all the reviews and scores I give are my opinions. I encourage you to post the score you would have given and the reasons why in the comments.
1: Category 1 casts a very wide net. Anything that is simply not worth your time makes it in here, from simply bad to truly offensive.
Examples: Sonic 2006, the Zelda CD-i games, Michael Bay's first Transformers movie (the only one I have seen and can therefore rate.)
1*: Anything that falls into a 1 category but has some unusual property that may make it worth watching. Usually these are things that fall into the “so bad it’s good” category or are historically relevant.
Examples: “Trapped in the Closet” by R. Kelly, Trolls 2.
2: Things that are ok. Generally speaking 2s are not worth seeking out but they aren't really bad either. If you like the genre or medium and material is scarce, you may want to check it out.
Examples: GI Joe: The Rise of Cobra, Michael Bay's Armageddon, Zombie Driver.
3: Generally good. Worth your time, especially if you enjoy the particular area that the thing falls into. A 3 is always a good score and is never an insult.
Examples: Gladiator, Super Mario Bros, Super Meat Boy.
4: Excellent. Highly recommended and must view for those who enjoy the category. Among the best.
Examples: Super Mario Bros 3, Half-Life 2.
5: The best. Very few things ever make it to category 5 and even fewer stay.
Examples: Super Mario World, The Shawshank Redemption, The Dark Knight.
Scores can and do change over time. For example, I don’t put anything in category 5 until it has sat for a few years and I see that it’s value remains over time. On the other hand, something like the first Super Mario Bros would have probably received a 4 at release but has lost points over the years as better things have come along.