What is your must grow crop for polytunnel?

I always put my butternuts in the tunnel, planted between the taller plants as a sort of ground cover. They don't crop very well outdoors up here, but always do well in the tunnel

I grow toms, cucs, aubergines, peppers, chillies and melons as you but would never grow tomatillos in there, they do really well outside and get huge there so no need. At the moment I have early potatoes, carrots (only just sown), lettuce, chard, spring cabbages, calabrese (planted last autumn and cropping now), dwarf peas, mange tout and broad beans. I grow small amounts of most of these crops under cover to extend the season of the main crops outdoors. Don't just think of it as a summer thing, I think it's most invaluable to use to extend seasons. Oh and don't put a courgette in there unless you want to fill the whole tunnel, they get huge and produce stupid amounts of crops which grow to marrows overnight - done that a couple of times and goodness knows why I repeated it a second time! Personally I wouldn't grow any squash in there and chose ones that do OK outdoors round here but if, like SarzWiz, you really want a particular variety then make sure you allow lots of space for them to spread.

Tomatoes for me, instead of tomatillo.

Also Sweetcorn for the first-early crop, followed by outdoor, I plant some Peppers and Aubergines between them having first grown them on as long as I can in pots, and then when I cut down the sweetcorn plants the Peppers / Aubergines follow-on.

In Autumn I plant Dwarf French Beans (to extend the outdoor season) and Chard (as a Spring crop), and sometimes more Dwarf French Beans in the Spring - I plant them at wide spacing and plant the Tomatoes between them and then when the Tomatoes need to space I pull the beans.