Depends. The xxx8 series are differentiated from the base models in different ways. For instance IIRC, 1628 has lightscribe over 1620, 1648 has solidburn over 1640, 1668 has lightscribe and solidburn over 1660 which just has solidburn.
If you really want a Philips, I'd say buy the 1660. It's the newest, has solidburn, and seems to be getting good reviews in both this incarnation and as a Benq 1550. If you want Lightscribe, then get the 1668.
In fact, you may not have any choice in the matter as I don't think 1620s, 1640s, and maybe 1648s are being made anymore. I just sent back a 1640 for a warranty replacment, and Philips sent me back a 1660. I've not played with the 1660 yet, but I've been using the same drive in the form of a Benq 1550 for the last few weeks, and it's a good drive.