The first five albums are all pure gold. If you have the funds, I suggest getting
Live at Carnegie Hall. Most songs are pretty faithful to their studio versions, but with some extra stuff here and there, and then there are the songs which are the bases for the extended jams.
If you can't afford it, or can't find it, I recommend either the first album (
The Chicago Transit Authority) or
Chicago V, which is my favorite studio album. The sprawling, semi-proggish arrangements of the first three albums (which were all originally double LPs) were scaled back into more concise jams, but they'd started playing more with time signatures, and Pankow's horn arrangements were getting really crazy. The playing on that album is both very tight and very loose; it's hard to describe.
The second album (either
Chicago II, or simply
Chicago, its original title) is also very good, but all the multi-song suites with little Roman numerals seemed like they were trying just a little to hard to be proggy, and that was initially a turnoff. Now I just listen and don't worry about it. All the sections segue, so it's not an issue.
Chicago III is good, but IMO the weakest of the bunch. It has some great stuff, and a few misfires as well. Seven guys in the band, and clearly there was a policy that anyone could submit a song for inclusion on the album. This had mixed results.
The Change
Chicago VII is probably their most adventurous album. It was originally going to be a jazz album; the band wanted to make an album that explored their jazz roots more properly, but the suits wouldn't buy it, so they compromised. It was another double LP, but there are no vocals until halfway through Side Two. It's all instrumental up to that point. This album was also the turning point for the band. Peter Cetera had the high tenor voice which was becoming so popular in the late 70's and into the 80's, and the three hits from that album ("Call on Me", "Wishing You Were Here", and "Searching So Long") all featured Cetera on vocals.
Chicago VIII had "Old Days", again with Cetera on vocals, and by
Chicago X (
IX their first Greatest Hits album) and "If You Leave Me Now" it was all over. Formerly one of three lead singers with approximately equal duties, Pete Cetera was now recognized as
the voice of Chicago.
It didn't help that guitarist Terry Kath died after
Chicago XI, taking the heart and soul of the band with him. Through the 80's and into the 90's, Chicago was basically a pop machine. Even when Cetera left to pursue his solo career, they replaced him with a bassist who sounded just like him, mostly because at that point, they needed someone to sing all the Cetera hits when they played them live.
So I'd say the turn started with
VII, but
VII is still an excellent album. It's how things played out over the next couple of albums. There are at least a few gems on everything up to
XI, but after that...