My first was MCO-DTW on a NW 753 in 2008. We booked late because of some schedule changes to my cousin's wedding, which happened to be around spring break in Florida, so FC was all that was available for miles on the return trip. I was just happy to be taking the long way (to Jackson, MS via DTW instead of MEM).
Best first class memory was springing for D1 on the Delta 744 roundtrip DTW-ATL during Thanksgiving week 2016, shortly before it was retired, and visiting the Delta museum while there. My wife and I got to experience row 1 (the nose cone) outbound and the upper deck returning.