Trans fat has been given the glorious title of "worst fat" but saturated fat (animal fat, dairy products with fat) is still dangerous artery clogging fat.  Polyunsaturated fats like vegetable oils lower LDL and total cholesterol and omega 3s (within that group can also help to raise HDL - the good cholesterol).  Monounsaturated fats like olive oil raise HDL and lower LDL and total cholesterol.  Beware omega 6 fatty acids (typically you'll find them in the form of linoleic acid) which are also found in a lot of processed foods - since they have been implicated in raising the risk for certain cancers due to the inflammation they cause. We seem to be eating too many omega 6 and not enough omega 3 fatty acids - which is why experts are recommending that we all eat low mercury fish like salmon 2-3 times a week.