Usually used is Cat5e, widely available, cable is cheap, wall sockets (RJ45) & patch panels are easily available and moderate in price.

Option is Cat6. One would guess that Cat6 is better investment for the future. cable itself seems not much more expensive than Cat5e and also relatively easy to find in many places. Cat6 wall sockets and patch panels on the other side are quite difficult to find. In Europe Cat6 hardware is considerably more expensive than Cat5e.

GBit-Ethernet (1000B-T, 802.3ab) is specified to work over Cat5e, which has a maximal transmission frequency of 100MHz, so for GBit-Ethernet there is no need for anything else than Cat5e.

Besides that Cat6a might be very difficult to find, it only makes sense in conjunction with wall sockets & patch panels of the same standard. Installation is much more difficult, it has seperators in the cable which keep the 4 pairs apart, which makes the cable 8-9 millimeters thick, minimal installation radius is 4-8 times the cable diameter!

Furthermore, the conduits should be of metal, interconnected to each other with low impedance, cables should not be bundeled more than 24 cables together and cable ties should not be closer than 2 feet to each other, etc etc.

So a Cat6a (certified!) installation is the only one that guarantees to be ready for the next step up to 10GBps, but in many places it might be close to impossible to find a company who can really do it.

Conclusion:

No need for Cat6, since 1000B-T works fine over Cat5e and 10GBASE-T requires Cat6a.

Caution:

In many regions of the world, maily in less developped countries, cheap qualities of (so called) Cat5e & Cat6 can be found. Often those cheap products are fakes that are sold under the label or known brands (e.g. AMP, Systimax, etc.).
Buy Cat5e and Cat6 only from trustworthy suppliers that can guarantee that the brands they sell are no fakes. Those cheap quality cables often do not even fulfil the standards minimum requirements and often only the two pairs used for 10/100Base-T are of reasonable quality, meanwhile the other two pairs are of appalling bad qulity.