The most popular album is, by far, Dark Side of the Moon but the Wall was pretty well known too (especially since they made a movie for it). Although there are some very good songs on these, I find I'm tired of them and play them the least. The albums prior to DSOTM (especially the first ones) are all excellent examples of 60's psychedelica with folksy pop tunes. Meddle (two prior to DSOTM) has some great atmospheric songs and not as vocal pop. Wish You Were Here comes right after DSOTM and shows them taking synths and progressive themes to a new level. Animals is almost a wonderful side oddity in my opinion. I'm also a fan of Division Bell.
There's some definite evolution (as with most good bands) but, for the most part, if you like one or two of their albums, you probably will like most or all of them. My least favorite songs are of the spoken word variety, though most are there to tell some part of an overall story.