I mix it up, I used to ride on my own 99% of the time but now I've moved it's nearer 50/50 because the local club/riders are good.
Plan routes, targets, things you want to explore or just go out and ride till you don't want to anymore.
I still really enjoy riding on my own. Gives you a chance to "session" a section you find hard. Push yourself, or not, on the hills. Go on long rides at your own pace so no need to stop and wait just riding continuously. Or just sit in a field, on a hill, by the sea and take it in.
Riding with mates is very different but equally as fun.