Hi emarbe, It is quite difficult to divide these into formal or informal, but I would say 'all the best' is the least formal in your list, and 'yours' the most formal ('yours sincerely' or 'yours faithfully' would be suitable for a business letter), with 'regards' and 'best wishes' being somewhere in the middle (formal enough for an acquaintance and informal enough for a friend). If I were writing informally to a friend, I might use 'regards', but if they were a good friend, I'd just write my name with a kiss (Rachel x) but I often see 'love', 'lots of love', 'cheers', 'bye for now' and many others. Hope this helps Rachel

All of those are fine for an informal letter to a friend