If you have a bike (or anything else that uses allen keys) the Park 3-way Allen Key (4mm. 5mm, 6mm) is miles better than any other allen key set. It’s much easier to use, and you’re not opening and folding away keys. Because of the Y design you get far more leverage than with either a folding key set or individual keys, in a really robust tool.
There is a newer style of this Park tool with a larger plastic centre – it doesn’t feel half as robust to me, so buy an old-style Y set if you can.
There is also a version of this with 2mm. 2.5mm and 3mm keys – it feels pretty flimsy and the keys twist under pressure, so I wouldn’t recommend it.