Decent 2 or 3 man dome tent.
North Face is what I will be getting next but any brand will do to be honest. This is one area where, generally, the more you pay the more you get. Look for the tent to have an inner tent and a water proof outer tent which, ideally, you can leave attached when packing it up. This speeds up putting the tent up no end. Also, if going to hot areas, look for a tent with zips front and rear which you can leave open to get a bit of a breeze through (obviously make sure they have flyscreens!) I would recommend putting this up in your garden at home before you arrive on the campsite and make a tit out of yourself trying to figure it out with a dozen amused campers watching on. This will also give you a chance to turn a hose pipe on it to check it is waterproof. If it is not then you can take it back and remand a new one.