I will seconds Guards! Guards! it's what I always suggest to friends as a starting point and it leads more naturally into a coherent series as you follow Vimes and the city watch - as an alternative the witches are quite hilarious especially if you're familiar with Shakespeare (a particularly good one here might be Lords and Ladies). The Lipwig books are also quite excellent featuring a lot of most folks favorite characters but I think they work better if you're familiar with the setting.

Lastly, I think a really good shout out is Monstrous Regiment which is absolutely hilarious, especially if you don't know the characters and just incredibly wholesome.