They're ALL productive if given the right foodplant etc - depends on the size you want...
There's not really a lower size for these because if you want to feed the lower instar mantis nymphs, just pick out a hatchling caterpillar...
Easiest in my opinion:
waxmoths - feed on honey, wheatbran/germ and a little glycerine
advantages: no foodplant, adults don't feed
disadvantages: glycerine is a pain to find, mixture CAN go off, short adult lifespan, need a little preparation to get them reproducing
silkmoths - feed on mulberry
advantages - very easy to find online, adults don't feed, largeish larvae
disadvantages: dried mulberry media is a little pricey and substantially reduced size and health from larvae
Hawk moths - privet HM = privet and deaths head HM on potato/ tomato
advantages: large adults and larvae, impressive looking
disadvantages: adults I believe do feed (it's been a while since i had these), the deaths head MIGHT build up toxins (solanaceous plants), less easy to find
cabbage white butterflies - feed on cabbages - easy to find
advantages: medium sized caterpillars, easy to find food, medium adults
disadvantages: adults DO feed (diluted honey solution)
Peacock butterfly - feed on nettles - easy to find
advantages: medium sized caterpillars, easy to find food, medium adults
disadvantages: adults feed, nettles hurt