Volatile oils responsible for flavor and aroma are the ones that boil off. That's why we add hops late in the boil and at flame out...so we can get the aromatic qualities in the wort. The bitterness associated with hops comes from alpha acids that take longer to assimilate into the wort. Think of it as an inverse ratio of bitterness and flavor/aroma - more bitterness = less flavor and more flavor = less bitterness. Since we want both in most beers we balance the hop additions at the beginning and near the end of the boil to get the best of both worlds. Using different varieties of hops allows us to get what we need. Some hops are extremely good for flavor and aroma but impart a harsh bitterness. Some hops are perfect for bittering but lack in the sort of flavors and aroma that are desireable for some styles.
Hops are pretty style-specific. When it comes to American hops and styles that are very hop-forward like IPAs and Pales, it'll come down to which ones you enjoy brewing with and drinking. Plenty to choose from.Now to figure which hop and when
That's a great method for IPAs and Pales, etc...If you're making a lager, especially something like a Czech Pilz, the balance of malt and hops requires getting the bitterness right without overpowering flavor and aroma from fragrant hops.Having said that, a local craft brewer recently told me that they almost never add any hops during the boil, their additions are almost exclusively whirlpool and or dry hops.