I love Waitrose frozen whole leaf spinach (keeps its shape better then chopped, which gets EVERYWHERE). Love frozen butternut squash chunks. Love frozen herbs, garlic, ginger, chilies. I prefer the ones from Waitrose (Cooks Ingredients) to Sainsburys because the Sainsburys ones come in boxes that I find difficult to remove the herbs from once they've been thawed and re-frozen (you know it happens to all of us). The Waitrose ones come in bags and they are much easier to remove - I can get a (clean) hand in there to remove stuck-together chunks if I need to..
Haven't tried frozen onions, rice or mash - I tried freezing my own mash once and it came out watery - don't know what I did wrong.
I was really proud of myself recently because I took a Jamie Oliver recipe for chickpea curry and totally "store-cupboard-ised" it. The recipe called for fresh tomatoes, a knob of ginger, cloves of garlic, fresh spinach etc. and everything I used was either from a tin or frozen (except maybe the onions). It was gorgeous! I now make sure I always have the ingredients on hand, which isn't hard.
I think frozen ingredients are probably better than packets, as frozen ingredients don't need to be messed around with to preserve them, while if ingredients are going to be stored at room temperature and not canned you have to put all sorts of garbage into them.
A relatively new cooking cheat: For roasties and chips I microwave the sliced potatoes for 9 minutes in a glass bowl, take them out of the microwave and bash them around, then put them back in the microwave for another 5-6 minutes and bash them again before mixing with oil, seasoning and whacking them into a hot oven for 30-45 minutes, depending on size. It takes as much time as parboiling but I'm not watching a boiling pot and can get on with other prep. It's also something DCs can help with. It's also more energy efficient, isn't it?