I would have to say a v7 is probably one that has not been mentioned as much as far as probs go but honestly there is a philosophy of mine I like to consider when I answer this question for you.
Lets use a major car dealer as an example. First of all the real quality of any given model depends on the assembly line. If John Doe comes to work on mind altering substances/alcohol or falls asleep on the job. The quality goes down the tubes. The same for a Doctor who goes through med school but partied all the time, just berely graduated, and now practices as a pediatrician....Now good ole Doc is treating patients even though he berely made it through school. Blah Blah Blah etc.......Does this really make every car/Doctor bad? Not really so actually you can get any model Ps2 and you may never have a prob ever, no matter what model. You could buy a v9 or 10 and never have a laser issue or you could be buying that 10% (bad quality) There is no real way to know exactually. Just take a leap of faith and hope for the best.
I guess in a sence, you really cant fault any certain model at all.
I still say the v7 has the least amount of probs that I have read so far though.