The SFWA named him a "Grand Master" not too long ago and that was one of the cases that somewhat surprised me, but I didn't think it diminished the list or anything. You don't exactly think "Asimov, Clarke, and Harrison" but he's written a lot of good and well-known stuff that's had an impact and it's likely they were also taking into account that he was also a significant illustrator and important editor and his overall contributions were impressive.
Some have said that he primarily wrote funny stuff but I don't know that that's true. His most famous stuff is funny but I'd say the bulk of his writing was serious.
That said, I've read and/or have about half and half. The specific titles (most or all of which have already been discussed) are:
The Adventures of the Stainless Steel Rat ((1961, 1970, 1972) 1977)
Still my favorite. The first three very funny galactic adventures of the famous crook/cop.
Bill, the Galactic Hero (1965)
This was okay and is generally discussed as equivalent to SSR but I didn't like it anywhere near as much and the later SSR books are genuine sequels (or prequels) whereas, while I haven't read any, I understand the sequels to these are basically sharecrop stuff.
Make Room! Make Room! (1966)
I also don't like this as much as a lot of people seem to but it's still quite good - one of the more famous late 60s/early 70s overpopulation books.
Star Smashers of the Galaxy Rangers (1973)
This, on the other hand, is vastly
underrated. As
This Is Spinal Tap is to metal (a brilliant parody that halfway works as metal that metal lovers (and haters) can love), this sendup of super-science space opera works as both satire and space opera that space opera lovers (and haters) can love.
West of Eden (1984)
Great idea of a sort of alternate history of dinosaurs evolving into sentience and having a highly developed but biologically-based science, with us monkeys wandering around, trying to get by, but I just didn't really love the book and never read the sequels.
A Stainless Steel Rat Is Born (1985)
The sixth SSR written and the first prequel. It was fine, as far as I recall, but I didn't get the other two sequel/prequels.
Stainless Steel Visions (1993)
A collection, which I discussed on
the short story thread.
In the pile:
The Deathworld Trilogy ((1960, 1964, 1968) 1974)
A significant serious series. While not unanimous, the comments on this thread make it sound like I wasn't mistaken in being willing to explore these.
The Stainless Steel Rat Wants You! (1978)
The Stainless Steel Rat for President (1982)
The last two sequels to the first three before he wrote the prequel trio (and then another sequel trio). I haven't read the original trilogy in a long time but I did read it twice, once very young and once slightly less so. I saw these cheap and good, so picked them up with the plan to re-re-read the first three and go on with these.
As far as things to get, based on this thread and other opinions, it sounds like he's written some other good stuff but none of it sounds like it's quite my kind of thing, so those might cover it for me: a collection, and then
Star Smashers and the
SSR series for the funnies, and
Make Room and hopefully the
Deathworld series for the serious.