Well, so far, here's what I've had:
DVD+R discs burned on the Ricoh wouldn't play on one of the DVD Players, the DVD player isn't supposed to play DVD+R (per the specs) but seems to play with some newer DVD+R media. This I would not qualify as my worst drive, because I am certain it was media issues now that I look back on it, even with bad media, it did pretty well.
The Plextor PX-708A suffered many firmwares where 8x writing on 4x media would be slowed at the end of the disc, and some discs burned were bad burn quality, unreadable, or, a flat out coaster. Otherwise, it was a solid drive, however, this drive does qualify for my worst drive.
The Pioneer DVR-107 was horrendous. The DVR-107 popped out more coasters than any of my other drives, and much of the burns were unreadable. This drive does qualify for my worst drive spot. This drive also has a very noticeable, and large bias for DVD-R media, burning DVD+R media, well, generally poorly.
The Plextor PX-712A has so far been excellent, 8x (or 6x) writing is flawless, where as it was flawful on the DVR-107 and the PX-708A. 12x writing also is flawless. Coupled with the 8MB buffer, which I give credit to for making higher speed writes flawless (unlike the DVR-107 or PX-708A.) I am also able to check the media quality, which seems much easier, and more reliable than the equivalent testing on Lite-On drives. This drive does not qualify for my worst drive spot.
So, who gets the final spot for the worst drive I have used? It's a close one between the DVR-107 and the PX-708A (which neither was all that bad anyway.)
I would have to go with the DVR-107 because it popped out a higher percentage of coasters, and unreadable discs than any of the other drives, however, the PX-708A follows in close second place for worst drive I have used.