I love Rochefort 10, Chimay (to a lower degree), and some other belgian beers. So I would like to try to do something similar, or just take some inspiration from them. What are the most commonly used hops for belgian trappist or abbey beer style?
Anything "Noble" that's not a typical American hop like Cascade/Centenial/Chinook/Columbus etc. I'd use Hallertaur, Spalt, Tett, Vanguard, Perle, etc. Basically any of the classic German hops, or their modern equivalent.
Honestly in those styles, you just want a touch of hop bitterness to cut the sweetness of the malt. Not many of those beers present much hop flavor. I understand that some styles (like Tripel) are actually a little hoppier when consumed relatively fresh in Belgium, but if you are trying to duplicate the taste of an older, imported bottle, then keep the hop flavor barely detectable and the IBU's in the 10-20 range.