Paragon levels have many benefits for your character, not only does it up your base stats (str, dex, vit, etc.) but it also boosts your magic and gold find by 3% for each level so by the time you reach paragon level 100 you will have 300% magic and gold find not even including your gear or NV (nephalem valor) stacks!
After patch 1.04 when paragon levels were first introduced the first person to reach paragon level 100 was a barbarian that went by the name of Alkaizer. Alkaizer figured out the most efficient way of gaining paragon levels while maximizing your drops along the way.
Some people have said the best way to gain levels is by doing a full Act 3 clear because all monsters in Act 3 will be monster lvl 63 giving them the most experience for the work. However this takes a long time to first get your stacks and then to actually get to areas with good enough groupings to get bonus exp. Alkaizer used this method to very quick gain millions of exp while still being efficient.