There are more than a couple of bands that I really like their first albums. Montrose and Bad Company both had excellent freshman and sophmore efforts, but clearly the first albums excell. They are new. They are excited. They are reaching out in their performances, and they are playing together on these first albums.
Lots of groups had great first albums and then faded into obscurity, either quickly or slowly. Bread was a band that released a killer first album in the early seventies, containing the most excellent tune "Mother Freedom", but the later albums to me were disappointing. Guns and Roses is another example. Appetite for Destruction was awesome, but subsequent efforts fell way short. Or take Janes Addiction. Their first indy album, titled Triple X, was a mostly live effort capturing them in their hungry pre-fame L.A. club days, and it is certainly one of my favorite albums. But thereafter, after they got the record contract, subsequent releases, although good, again felt contrived and over produced and just didn't rock like XXX did. And likewise with other artists.
Of course, there are exceptions to this rule, like almost every rule.
The Beatles. The Stones. The Who. The Yardbirds. John Mayall and the Blues Breakers. Led Zeppelin. The list goes on and on of super groups and artists that arguably had their "best" album(s) later in their career, although usually great debates occur with fans as to which album is truly the greatest.
I'll think of some other First Album Bands I know of and list them in an edit here, and as always you're always welcome to comment and make me smarter.