Burning at the slowest possible speed will NOT in general give you the best quality; in some cases you might even get the worst possible quality by burning high-speed media at the slowest possible speed.
The optimal burning speed depends on the media, the drive and the firmware.
For a broad range of media, drives and firmware however, I have found by burning and testing that 8x is the safest burning speed for 16x and 8x media. There are exceptions but they are rare. For some drives it's better at 6x or even 4x and in very rare cases it's better at 12x.
If you want a very simple rule and you don't want to burn and test yourself, then I suggest burning 16x and 8x DVD±R Single Layer media at 8x on a desktop drive and at 6x on a notebook drive.
For CD-R media a very simple rule is to burn Audio CDs at 16x and Data CDs at 32x in a desktop drive, and burn at 16x in a notebook drive.
Pioneer DVR-112L, LiteOn SHM-165P6S, Pioneer DVR-111L, BenQ DW1655, Plextor PX-760A
LiteOn LH-20A1P, Optiarc AD-7203A, Matshita UJ-840S, Sony RDR-HX910, Sony RDR-HX750