depends on how much you want to spend and what you want the lawn to do or look like. is it a kids play area, a piece of grass to look at are you embarrassed by the neighbours lawn looking better than yours etc.
If it is shaded use a specially selected mix for shaded lawns. If you want grass for children to run around on use cheaper ryegrass.
sometimes it is how you treat the lawn rather than the seed mix that leads to patchiness.spring is not the best time to reseed as the young grass cant get established before the summer brings (hopefully) dry weather.Early autumn is best time to establish grass.
Hope this helps.