Out of all the gear what is the piece that gets serviced the most?
Anything with batteries has been my experience. I normally change at the beginning of the diving season, and depending upon usage, some equipment may require additional changes throughout the season. Regulators may require annual maintenance for warranty purposes (free replacement parts), other manufacturers may go two years for their service intervals. Straps (fin, mask, snorkel keep) should be in your fix-a-dive kit meaning that Professor Murphy will pay you a visit when you are least prepared/expecting. Also, the care that you provide (fresh water rinse, cleaning, dry storage) can have a profound effect upon your equipment's need for professional servicing. Your life depends upon reliable, safe dive gear. Make it a habit to keep it in great shape.