Farmhouse is a good rec. My old blues father-in-law has been a professional musician for 40+ years now and hates Phish and Grateful Dead for some unknown reason, like many people.

One day, he came over and we just happened to be playing Farmhouse. I wasn't about to take it off just because he showed up. I noticed he seemed to be into the music right away and about 20 minutes later, he said, "man, this is pretty good, who is this?" and I said "Phish" and his face dropped and he said "you did that on purpose." Which still makes no sense to me. Like, yeah, I intentionally had them record this album so I could trick my inlaw into liking it?

Anyway, Farmhouse is the one album a lot of people generally like. I honestly don't understand why people don't like Phish. Makes zero sense to me. Every album is unique in some way, but the other albums that seem sort of "mainstream" like Farmhouse to me would be The Story of The Ghost, Billy Breathes, and Hoist. If you like those, then try out Lawnboy. If you still don't hate Phish, then I would recommend A Picture of Nectar and if you want more still, then Junta might just blow you away. Junta was their first studio album and, to many people, it seems super nerdy at first, but once you get an idea of what Phish is all about, it may just be your favorite of them all. Finally, check out Rift. Rift is a big favorite and ramps up the geekiness with prog-like musical twists and turns. Kind of a dark album, definitely a sinister vibe with barbershop quartet style harmonies and cartoon zaniness thrown in. This was my least favorite for quite a while, but it's many a Phish-heads' favorite. I also didn't like Billy Breathes very much, but most people think that's the best album they ever made.

Anyway, good starting rec from /u/Jam_flow_man.