It's not so much a matter of one mount or one brand that will give you excellent lenses and another will give you junk - it's unfortunately on a lens-by-lens basis. There are some gems in almost every mount, and it sometimes just takes a bit of experimenting or searching around to run into the good ones. I have about 25 manual lenses I've collected over the past 9 months for my NEX3 - the most I've spent on any of them was $17. Some have been 'average', none have been 'poor', and a few have been excellent. When I find a lens or lenses I'm interested in, I search around the forums and internet for some input on the lens from anyone who has one or has used one - sometimes it's an old lens review from film days, sometimes it's a current user with a NEX or M4:3 body on one of those forums or rangefinder forums...if the lens gets mostly poor reviews, I'll pass. If it gets decent to good reviews, I'll bite, assuming the cost is low enough where I have little to lose.
Some of my gem lenses I've picked up cheap have been:
Chinon 135mm F2.8 (Pentax K mount) - superb sharpness, lovely bokeh, very low CA, mild PF only in extreme contrast situations, nice contrast.
Vivitar 200mm F3.5 (Konica K-AR mount, Komine build) - same as above - sharpness is excellent, likely exceeding the sensor, while color and bokeh are pleasing and no CA issues.
Konica Hexanon 50mm F1.7 and 40mm F1.8 (Konica K-AR mount) - two lovely primes, nice and fast, good sharpness, very nice stopped down a bit, slightly dreamy/diffuse wide open, no CA issues to speak of.
I generally like Konica's Hexanon lenses, and many of the Vivitar branded lenses can be quite excellent, if you stick with those built by Komine especially, or sometimes the Cosina or Tokina builds (which you can find out from decoding the serial numbers). Since these were built for many different mounts, you can just look for the lens, and pick up the adapter needed to mount to the NEX.
Sony DSLR-A68 / Sony 18-250mm / Minolta 50mm F1.7 / Tamron 150-600mm / Minolta 300mm F4 APO
Sony A6300 / 18-55mm F3.5-5.6 / 55-210mm F4-6.3 / 10-18mm F4 / 35mm F1.8 / 16mm F2.8 / FE70-200mm F4 G OSS / FE70-300mm F4.5-5.6 G OSS / via manual adapter, lots of Pentax K mount, Konica K/AR mount, and Leica M mount manual lenses