I read Baby Proof and Heart of the Matter, in that order, but wouldn't suggest saving Heart of the Matter for last; out of all her books it was my least favorite. Don't get me wrong, I love ALL of her books, but if I had to put them in order I would probably go with:
1. Baby Proof
2. Something Borrowed
3. Something Blue (you have to read this directly after Something Borrowed. period.)
4. Love the One You're With
5. Heart of the Matter
The only thing I might reserve the right to change is placing the Something Borrowed and Something Blue series first, but that would be it. Honestly, if you like a single one of novels then you'd like them all. Don't take my word for it, read for yourself.