So far I've read Guards! Guards! and Reaper Man.
The 4 next books I have are: Sourcery, Witches abroad, Night watch and Men at arms.
ack you've missed a couple of books out of the guards series >.<
Theres a long continuing story line in this series within a series which goes something like this.
Men at arms,
Feet of Clay,
Jingo,(this ones a little slow)
The Fifth Elephant,
The City Watch is definitely my favourite story arc. I do recommend reading them in sequence. The stories aren't connected as such, but things happen in each story that have an impact on the world, so if you read them out of sequence, you'll likely be puzzled as to why things are as they are. They are the most serious of the Discworld novels, while still being very funny, and contain many great moments of awesome badassery.
Man, now I want to go read the Discworld pages on TVTropes.