Just throwing in my 2 cents.... dunno how you can have a "beach boys starter pack" and not include anything pre-pet sounds... that's as odd to me as oh, making someone a Metallica mixtape and deciding to exclude the Cliff Burton years, or making a Nirvana mixtape and excluding anything on Sub Pop. It is an incomplete picture of their trajectory and development and that's not what I'd want in a starter pack.
If it were me I would pick 4 albums. The first would be a solid pre-PS record like Surfer Girl or Summer Days!, then Pet Sounds, then a post-PS Brian-era album like Sunflower or maybe Wild Honey, then your favorite of the 3 Carl-era records.
On the other hand, if what you're interested in is actually a "Brian Wilson starter pack" then that is different, and a good 4 would be Today!, Pet Sounds, Friends, and Love You. (Then again, I really, really love Today!)
Interested listeners could likely find major differences between each of the 4 records, and dive in from there and further explore the era surrounding their preferred LP.