What is the nicest a Unix command can be?

For a quick benchmarking test, how can nice and ionice be combined to maximum effect, ie for a command to use as little resource as possible (without idling altogether)? *(I think it's something like `nice -n 19 ionice -c 2 [command], but not sure about ionice's "-n" (classdata param), the man page is cryptic about its relevance.)*