The Zombies were a thoughtful group of musicians in the 60s, who upped the scale of great pop music. Though they only made a few albums and a string of singles, they have a memorable collection with the tops of their era.
If you want the collection of the best of the best, I recommend this Singles Collection. If you want everything, I recommend The Anthology. The Odyssey and the Oracle is their art piece though you can get those songs on both these collections.
"She's Not There" and "Time of the Season" were their big hits.
Also, "Tell Her No" did well. These showed their vocal and musical complexity. Other songs that have great musicality are "She's Coming Home", "I Love You", "Remember You" (featured in the Otto Preminger film "Bunny Lake is Missing"), "She Does Everything for Me", "Friends of Mine" and "I'll Call You Mine".
I also recommend "Summertime", but you will find it on Anthology.
Colin Blunstone's vocals and Rod Argent's keyboard playing are the signature factors that make their music stand out. Argent, along with guitarist Chris White wrote most of their own songs.
The Zombies, which was a poor selection of a name for them and their music, were pop at a very high level - good musicians with thinker lyrics. They weren't really Mersey Beat, but certainly British Invasion.
For further interest, several years after they broke up, Argent came out with a band with his name and they produced the Zombiesque song - "Hold Your Head Up".