Although bock beers are a malt forward beer, they still use hops as do most beers. The type of hops have been held in a category that some call “Noble Hops”. These are low alpha acid hops that carry a distinctive flavor profile. Also the majority of these hops are German in origin.
An interesting point, while I was digging through Stan Hieronymous’ new book For the Love of Hops I ran across a note about the term noble as it pertains to hops. It is true that German beers used select hops in their production. Most beers use a certain variety of hop based on the region the beer came from. This is true throughout the world. But the term Noble hop was not used until the 1980s.
This tells us that the styles of Germany, including bocks, are an affect of their environment, not the other way around. The hop is a part of the terroir of the beer as much as the water source that the beer was brewed with. Malt is not as much a factor in the terroir because its character is determined by the maltster.
Of the hops used in German style beers, Hallertau is one of the most prominent. Hallertau is the varietal. There is a Hallertau region in Germany (where this hop originated) but this does not mean the hop was grown there. This hop tends to have a second designation to denote where it originated from. The most common being Hallertau Mittelfruh. This is also the hop that most Hallertau cultivars orginated from.
Another hop commonly used is the Saaz. This hop is Czech in origin. With the birth of pilsner and its adoption into German culture this hop gained life in German beers.
Tettnang Tettnanger, which is a member of the Saazer family is also used at times.
In the US common hops in these style beers have included Liberty, Northern Brewer, and Sterling. Each of these hops has origins with traditional German hops.
These hops all have the commonality of low to moderate alpha acid (the primary bittering oils). They have aromas and flavors revolving around floral, herbal, and spicy. You will not find the citrus or resin you might find in some US varieties (e.g. Cascade).
Essentially when brewing a malt forward beer, the hop selection you would choose for it should help to enhance the malt.
Time for a pint…