I love my Whistles tube skirts - but if I could only rescue one item, it would be a Vivienne Westwood Red Label skirt that is the most expensive item of clothing I've ever bought (£100 in the sale) and is very 'me'. I would have to replace my favourite pair of Converse and my tube skirts immediately though!
God that's a hard one! Possibly a sort of grey dress from Second Female, prob 3 years old now, short sleeved drapey thing at front, quite clingy but in the right places, can wear on it's own in summer but over skinny jeans too. Looks great with any shoes or sandals, evening or daytime. Sounds quite dull but is one of those fall-back things which I know I won't go wrong in. And irreplaceable now.
Or maybe a beloved Nicole Fahri long backless halter neck dress which you have to be size 8 and firm-titted to wear which I adore though not much occasion to wear it these days
I do like the Whistles tube and is definitely useful, but replaceable.
Ahh, I see you're going for the irreplaceable items. Ignoring those, I am thinking more what can you not do without on a day to day basis. I.e. Shit! my wardrobe's burning down, I need an outfit to be rescued in/ wear until I can shop.
* might need revisiting if especially hunky rescuer popped through the window.
Generally I would say an inferno wouls be the best thing for most of my clothes, but I do have a few Mantaray tops and dresses that I'm quite fond of. Assuming that it was a cold day, I could be wearing two of them thus requiring only one to be actually recued . And a very forgiving long cardy.