Here's what I've read:
1998-01 - Buffy - Coyote Moon
1998-08 - Buffy - Blooded
1998-10 - Buffy - Child Of The Hunt
1998-12 - Buffy - Return To Chaos
1999-01 - Buffy - Gatekeeper 1 - Out Of The Madhouse
1999-03 - Buffy - Gatekeeper 2 - Ghost Roads
1999-05 - Buffy - Gatekeeper 3 - Sons Of Entropy
1999-11 - Buffy - Sins Of The Father
2000-04 - Angel - Not Forgotten
2000-08 - Buffy - How I Survived My Summer Vacation
2001-05 - Buffy & Angel - Unseen 1 - The Burning
2001-06 - Buffy & Angel - Unseen 2 - Door To Alternity
2001-08 - Buffy & Angel - Unseen 3 - Long Way Home
2001-08 - Buffy - The Lost Slayer 1 - Prophecies
2001-09 - Buffy - The Lost Slayer 2 - Dark Times
2001-10 - Buffy - The Lost Slayer 3 - King Of The Dead
2001-11 - Buffy - The Lost Slayer 4 - Original Sins
2002-12 - Angel - The Longest Night
2003-03 - Buffy & Angel - Monster Island
2003-04 - Angel - Sanctuary
2006-01 - Buffy - After Image
2006-04 - Buffy - Carnival Of Souls
2006-10 - Buffy - Portal Through Time
I'm a Bangel and a fan of the high school seasons + AtS more, so my selections should be appealing. The Bangel in Child of the Hunt, Monster Island and the Gatekeeper trilogy is really good stuff with meaningful moments and quality interaction. My primary interest is Angel, so books that are heavier on him tend to go to the top of my list.
Here's a thread I made a while back where I discussed what I thought of what I had read so far:
BtVS/AtS tie-in novels | IMDB v2.0