I'm trying to make more of what I already have so looking at the most versatile/useful items I have and would love to know everyone's 'must have's' so I can steal ideas! Jeans are a given!!
I'll kick off...
Black ankle boots Took me ages to find a pair that suited - now I wonder how I dressed myself without them.
Plain grey Gap sweater - works with absolutely everything, even for work if I put a shirt underneath.
Grey H&M a-line above knee skirt I have worn this to work once a week, every week since I bought it!! It's getting a bit tatty TBH 
M&S blue cashmere cardi Warm (tick), Good fit (tick), flattering shape (tick), works for casual or work (tick), lovely colour (tick)
There are other items I wouldn't be without but those are the most worn items.