best for oily skins-try clinique, and mac, which out of all the designer brands are the cheapest at £19 for foundation (mac) and slighty above for clinique, also try estee lauder. Lancome also do a good matt foundation, but make sure you try on several different colours in natural day light, and clean your face before trying it out as trying foundation over foundation gives an untrue result.
Wear a primer under foundation-the primer doesbt have to be premuim brand. 17 from boots has an offer for their new primer-ive bought it and its great.
its always worth paying extra for premuim foundations-this is your face we are talking about and dont cut corners with your skin. They generally last a long time and look great on.
If you have oily skin you still need to moisturise, try vichy normaderm (£11 in boots) the range by avene, (avail in boots)-and remember oily ski is generally also sensitive so these products are good,