Ratcatcher
I feel compelled to compare this to two movies. Morvern Callar because it is from the same director, and Fish Tank because it focuses on a teenager living in a British slum (in this case in Glasgow, making it important that I say British rather than English).
It is, simply put, a vast improvement over Morvern Callar (which is to say, she got much worse by the time she made Callar). Unlike Callar, she gives us characters to sympathize with and thus an avenue into caring about the story. Unlike Callar, there isn't so much flashy, remedial camera choices (no blinking lights). And unlike Callar, the nudity here does not seem forced and sleezy, once being used as an indication of the poverty of the situation and another as a slight contrast to that poverty while also being a major aspect of the main character, James, coming of age. The grime in this, from the ditch running by the development that seems to be polluted to the lice in the characters hair, got me all itchy, so that seems another mark in its favor.
But Fish Tank stands out to me as the superior film here. It is a bit more energetic. As seems to be Ramsay's style, Ratcatcher seems a bit lethargic. I know you'll find a lot of fans of long, slow shots here, but I find it an irksome technique. I don't need a thirty second shot of someone walking down the street unless you've got a very good reason for it. Plus, while Fish Tank has an ample supply of heavy scenes, it also provides some lighter and more joyful moments (anytime Mia dances, for example). Ratcatcher, like Morvern, seems to pay no mind to this need for contrast and is content to just mire you in misery. One particular aspect of this is a group of bullies that only appears to cause some form of mischief or mayhem, but they are never developed in any way that makes them humans rather than monsters. That isn't something that works for me. I either like the background that provides explanation for evil, or I like to see evil punished.
Anyway, definitely much higher on Ratcatcher. It has enough things to keep me from loving it but the general concept bolstered by strong child actors makes it worth seeing and a much better entry point to Ramsay's work than Morvern.