Ideally I'm looking for 512MB RAM. Like @
MannDude said, it's big enough that I don't have to worry about optimization. Disk and bandwidth sizes don't matter that much to me since I never use much of either. As a minimum, though, in terms of disk I might say 10GB SSD, 20GB HDD. Gigabit port is nice, but not all that important. IPv6 is a
must. As for KVM vs. OpenVZ, I prefer OpenVZ for dev mostly due to its simplicity. One click OS (re)install and overall easy access to the container through SolusVM (including changing the password). Only thing is OpenVZ can't do a proper /64 of IPv6. Final thing is that the provider should have Ubuntu available - at least 12.04, preferably 14.04.
Exception is DNS servers. I have five, and I'm happy enough with them at 128MB. Some say BIND needs a lot of memory, but that's not true in my (admittedly limited) experience. I have one DNS server with plenty of memory in case an influx in traffic causes BIND to consume more resources, but most of my DNS boxes are 128MB. 100mbit is fine, Ubuntu 12.04 is preferred, disk space doesn't matter at all.