On one hand I'd recommend getting the "The Very Best of Supertramp" and "The Very Best of Supertramp 2" since they compile almost every good track from their 4 main albums. But on the other hand those comps are 80 minutes long and the albums are 45, so all you'd be getting with those comps is just those 4 albums with the tracks out of order. So you might as well just get the regular albums.
I will put in a good word for 1985's "Brother Where You Bound" which is effectively a Rick Davies solo album, but a really good one. Compilations and stuff often skip it. I actually like it better than some of their 70s stuff. It doesn't sound like Supertramp much really, but it's good! It's got a 17 minute prog epic with David Gilmour guitar solos.
Roger Hodgson's first solo album "In the Eye of the Storm" on the other hand sounds *exactly* like supertramp and is just as good as any of their best albums. It's essential for anyone who enjoys the band IMO.