Best Filler

[sg31]

If the crack is due to expansion amd more or less a staraight line you coud try covering it with beading. There are a variety of profiles you could use. Stick the beading to one side but not the other then it can move with the wall.

Beading is cheap, available at most DIY stores or builders merchants and can be painted the same colour as the wall. You might notice it but no one else will.