Usualy rare, but starting to change my ways. Wish I could find the original article I read. It very much depends on the cut of beef and what marbling/connective tissue it has.
Found this table, but no where near as informative as the original.
Tenderloin Steak
Tenderloin cuts are at its best when 'Rare' or 'Medium-rare'. Texture is soft and very tender. 'Medium' is still acceptable for those who are uncomfortable with rare beef. 'Medium-well' tends to be too dry and the texture is very coarse.
Ribeye Steak
Ribeye cuts are at its best when 'Medium' or maybe 'Medium-rare'. The slightly longer time on the grill enables the fat and interconnective tissues to break down and melt, giving it an intense flavour. If 'Rare' the fibres tend to still be a little chewy and the fatty tissues are not as flavoursome yet. 'Medium-well' are also acceptable for those who prefer them due to the higher presence of fatty tissues, but the texture will tend to be a little coarser.
Striploin Steak
Striploin cuts are best when 'Medium'. 'Rare' tends to be chewy. 'Medium-rare' is only advisable if the beef is of prime quality or a cut from a Wagyu.
Sirloin Steak
Preference is to have them 'Medium' or 'Medium-well' but best not to exceed a thickness more than 25mm thick. Using a mallet to pound it slightly would physically make it more tender.
Rump Steak
Preference is to have them 'Medium' or 'Medium-well' but heavily pounded with a mallet. 'Rare' would be very chewy (unless heavily pounded with a mallet).
Round Steak
Preference is to have them 'Medium' or 'Medium-well' but heavily pounded with a mallet. 'Rare' would be very chewy (unless heavily pounded with a mallet).