My life, Monstrous Regiment was quite quite awful. Almost as bad as Moving Pictures.
The Truth is excellent, as is Witches Abroad and Lord and Ladies. I'm a witches fan.
If its Witches you want then Masquerade is a great story and I think would work as a "First" witches story (rather than going back to Wyrd Sisters which is a bit 'dry'
Sir Terry has often said if you want a starting point then he would point to Morte , and that he has become a better writer as time goes on - if you have enjoyed "Going Postal" then "Making Money" is the next in that series (with the continuation of the Moist storyline)
The full list from Wikipedia - Night Watch is probably my favourite but it helps if you've read the other Guards ones to get the background
Some of the Standalone ones are also very good , I enjoyed "Pyramids" , and "Small Gods" is an interesting story with only 2 'recurring' characters
ZOMG Spoiler! Click here to view!
Death and The Librarian <ook>
1 The Colour of Magic 1983 Rincewind Came 93rd in the Big Read.
2 The Light Fantastic 1986 Rincewind
3 Equal Rites 1987 The Witches, The Wizards
4 Mort 1987 Death Came 65th in the Big Read
5 Sourcery 1988 Rincewind, The Wizards
6 Wyrd Sisters 1988 The Witches Came 135th in the Big Read
7 Pyramids 1989 Miscellaneous (Djelibeybi) British Science Fiction Award winner, 1989
8 Guards! Guards! 1989 The City Watch Came 69th in the Big Read
9 Faust Eric 1990 Rincewind
10 Moving Pictures 1990 Miscellaneous (Holy Wood), The Wizards
11 Reaper Man 1991 Death, The Wizards Came 126th in the Big Read
12 Witches Abroad 1991 The Witches Came 197th in the Big Read
13 Small Gods 1992 Miscellaneous (Omnia), The History Monks Came 102nd in the Big Read
14 Lords and Ladies 1992 The Witches, The Wizards
15 Men at Arms 1993 The City Watch Came 148th in the Big Read
16 Soul Music 1994 Death, Susan Sto Helit, The Wizards Came 151st in the Big Read
17 Interesting Times 1994 Rincewind, The Wizards
18 Maskerade 1995 The Witches
19 Feet of Clay 1996 The City Watch
20 Hogfather 1996 Death, Susan Sto Helit, The Wizards Came 137th in the Big Read; British Fantasy Award nominee, 1997
21 Jingo 1997 The City Watch
22 The Last Continent 1998 Rincewind, The Wizards
23 Carpe Jugulum 1998 The Witches
24 The Fifth Elephant 1999 The City Watch Came 153rd in the Big Read; Locus Fantasy Award nominee, 2000
25 The Truth 2000 The Ankh-Morpork Times, The City Watch Came 193rd in the Big Read
26 Thief of Time 2001 Death, Susan Sto Helit, The History Monks, The Witches Came 152nd in the Big Read; Locus Award nominee, 2002
27 The Last Hero 2001 Rincewind, The Wizards, The City Watch Published in a larger format and fully illustrated by Paul Kidby
28 The Amazing Maurice and his Educated Rodents 2001 Miscellaneous (Überwald) A YA (young adult or children's) Discworld book; winner of the 2001 Carnegie Medal
29 Night Watch 2002 The City Watch, The History Monks Received the Prometheus Award in 2003; came 73rd in the Big Read; Locus Award nominee, 2003
30 The Wee Free Men 2003 Tiffany Aching The second YA Discworld book
31 Monstrous Regiment 2003 Miscellaneous (Borogravia), The City Watch, The Ankh-Morpork Times The title is a reference to The First Blast of the Trumpet Against the Monstrous Regiment of Women
32 A Hat Full of Sky 2004 Tiffany Aching, The Witches The third YA Discworld book
33 Going Postal 2004 Moist von Lipwig Locus and Nebula Awards nominee, 2005
34 Thud! 2005 The City Watch Locus Award nominee, 2006
35 Wintersmith 2006 Tiffany Aching, The Witches The fourth YA book.
36 Making Money 2007 Moist von Lipwig Locus Award winner, Nebula nominee, 2008
37 Unseen Academicals 2009 The Wizards, Miscellaneous (Nutt)
38 I Shall Wear Midnight 2010 Tiffany Aching Fifth YA book