this one is both hard and easy for me to answer.
my first serious watch, a breitling chrono avenger, was bought with a small legacy from my departed maternal grandmother in 2001. i didn’t expect to be a collector at the time, and wore that watch daily for about eight years. my second good watch was an omega SM PO 2500, which i added because i couldn’t afford a submariner at the time. the seamaster was also purchased with funds from a legacy from my paternal grandmother.
both of those watches are seriously sentimental. and i’ll never sell them.
buuuuuut.
in 2015, while in vienna with my stepfather and mother (it would prove to be his final international trip, as he began to suffer more from parkinson’s symptoms after that adventure), i was at Juwelier Wagner on stephansplatz and had tried on a batman. i loved it, and i wanted it to be my first rolex - i had finally come around to understanding what rolex was and why they were rolex. i went to pay the woman, and she smiled and said “that won’t be necessary, the gentleman has already settled your account.” my stepfather was a quick one, that guy.

so my first rolex, bought for me by my stepfather on his last international trip. heavy. more wrist time than anything in my collection since.
and now..: the watch i’m wearing daily, the 228206 meteorite diamond. bought to memorialize that same stepfather, as he was as rare as they come.
i have sentimental attachment to many of my watches but these two rolexes definitely take the cake.