In addition to Amandeep's answer I would say that it also depends on how you expect users of your service to use the saved items. Is it possible that a user has a range of prepared queries saved up that searches for different things, or is it more probable that a user has one (or two perhaps) single search items that they constantly revisit to update for new types of searches?
If the answer is the former, then users will probably have a range of search items prepared that are titled according to what type of information the item will search for. In that case a user will find it much easier to relocate a specific item if the list is alphabetically ordered rather than ordered by last-viewed.
To give an answer that could be considered "correct" you need to know what the majority of users will look for when opening the list of saved search items.