Splenda - it's the easiest to use in cooking and in drinks, unlike other sweeteners it doesn't form a skin, and again, it's non-aspartame which is a huge bonus.
The other sweetener I use is from Aldi's yellow-top sweetener (Tesco also sell exactly the same one, in the same container, just a different label and a higher price) which is sucralose based and a very good Splenda substitute.