If you are sure that all your existing and possibly future routers are going to be Cisco's, then I'd recommend using
EIGRP as the routing protocol. EIGRP supports VLSM and in general is easier to configure than OSPF. OSPF will work fine too, but it's a bit heavier on routers in terms of CPU and memory usage.
However, EIGRP works only on Cisco based routers. If there is a chance that your going to have routers from other vendors (like: Juniper or Foundry) in the future then EIGRP is out. In this case, OSPF will be your best bet.
I have to repeat that OSPF can work well even in the first case and has it's own advantages too.
www.ncat.co.uk/cisco-documents/ospf-information.htm
RIPv2 is not recommended for such a large network.
