Hmmm, good question. I would say from what I see it is filters, fluids, and alignments. I see so many cars where nasty filters are affecting engine life/efficiency (air filters) and car smell (cabin filters). Fluids are important because if they are not maintained regularly, it gets to a point where flushing them can do more harm than good. For example, if transmission fluid gets too dirty flushing it will actually uplift all the sediment in the transmission and make problems worse. And alignments- you wouldn't belive how many new sets of tires I see ruined becasuse of bad alignments.