What do you want to study at University/in college?
If you have a faint idea (maybe you don't have a particular degree in mind yet, but you might know what A levels you want to take) then I'd work your work experience around these. This way, it will be relevant for your Uni application. No point in knowing that you want to study Law and spending 2 weeks in IKEA.
And in regards to "what placement would produce good results in return for hard work in the form of a written statement from my supervisor", it doesn't really matter what the work experience is, but if your supervisor gives you a glowing reference saying you were punctual, committed, had great organizational skills, a good appearance etc. then this is what makes the reference from work experience good.