All of their officially released live shows are excellent, so those are easy go-tos if you want to sample different eras. The original Live Phish series is a pretty good place to start for highlight shows. there are 20 volumes of that, and just about any of them are good places to start. The band has shaken up their sound a few times in various ways so it's worth sampling shows from different eras. Early 90s were more song-driven with super tight playing, lots of songs weaving in and out of each other and lots of their humour and in-jokes. mid-90s is soaring upbeat jams, late 90s is more loose with lots of funky grooves and shows where the jams eclipsed the songs, early 00s has more spacey jams. Current era is kind of a little bit of everything.
If you want studio albums Junta, A Picture of Nectar, and Rift are the best starting points.