Damn I feel old.
Chicago: Chicago IX (1975)
This was my first real exposure to Chicago. Oh sure, I was aware of the band (I mean you couldn't turn on the radio without hearing them), but this was the first album of theirs I actually bought. I remember putting the record on my turntable, plugging in my headphones and falling in love. From that point on I was a huge fan, and as recently as the 2000s I was still backfilling my catalog with the earlier albums I'd previously brushed off.
To be honest, however, I did lose interest in the majority of their work after Chicago X, the album that came out immediately after this one.