There really is no "best" drive, but I'm happy with my external LiteOn 1635S - it's not as good at burning CD-R as my NEC and Plextor drives however (don't know about my BenQ drive yet). So if CD-R burning is very important to you, there are better choices available.
Even though a 1635S is the LiteOn i have now, I would most probably buy a 165P6S instead if I had to buy the drive now, since the 6S series has more firmware capabilites.
There is one little problem you may want to know about the current LiteOn 5S and 6S drives. This problem exist in the released firmware versions, and has been solved by LiteOn in firmware that haven't been released yet - but will be released as soon as testing is finished.
The problem is a non-standard lead-in when burning DVD-R media, and this makes the DVD-R disc unreadable in some players/drives.
As I said, the problem has already been solved in firmware that will soon be released. But if you need to burn DVD-R media now, you need to be aware of this issue.