Coin collection, and especially ancient coins. I’m a far cry from the fat cats who could put a museum to shame with their Roman and Greek coins, but I’m fairly confident that I have the largest and most complete collection of Indo-Sassanian coins in private hands, or possibly in total. There are types that I own 60-80% of all known examples of.
Roman collection is coming along, although I can’t bring myself to cross the threshold of spending more than $1,000 on a single coin. My kids are going to have a field day with that when I kick the bucket.