If you want something that can work for more than one occasion, I’d go for a sleek midi with adjustable straps — you can dress it up with statement heels or keep it minimal with simple jewellery. I recently picked mine from
https://satinlondon.co.uk/ because they’ve got different black satin options, from open-back to high-neck styles, all in that smooth finish that catches the light just right. It’s worth browsing there since you can get something that fits both formal and relaxed parties.