Technically they are NOT all the same. Some manufacturers might just brand cat6 cables as cat5/5e so they don't have to support 2 manufacturing line, but that is another subject.
When we hear "quality", most of us think of the materials and/or the manufacturing process. For cat5/5e/6, the difference is in the maximum bandwidth the cable can support; not because the cable is "better", but simply because it was manufactured in a way to allow it. Electricity is a pain to work with, when you have 4 pairs of wires close together there is electromagnetic interference that must be taken into account; you can work with it or against it.
On a more down-to-earth aspect, cat5 is tested to support up to 100Mbps (~10 MB/s), cat5e 1Gbps (~100MB/s) and cat6 10Gbps (~1GB/s, supposed). If you just want to connect a cable modem to your PC or router, any would do since very few ISP offer even 50Mbps connections. If you were to connect a home network, I would go cat5e or cat6 all the way since file copy from PC to PC could reach ~50MB/s (500Mbps).