There are two finalists for this spot in my opinion.
Pickering is one of my latest blooming trees. This is good if frost is an issue at your location near or after normal mango bloom time. Not a good mango to pick mature green, best when they are about to drop and then wait a couple days before eating. Great productivity,tree health and compact tree footprint, good sized fruit with a broadly appealing flavor. Fruit after harvest can last longer than most without developing off flavors. A fruit that is just below elite status when properly picked and ripened , but overall a favorite of many and one of the best choices for a single tree. Pickering's harvest window and tendency to drop when almost ripe is it's only shortcoming. Average season for this mango for me is about 6 weeks,but they start to drop as they get ripe so they won't hold on the tree.
Little Gem is my choice if you only have room for one. Also late blooming. Small tree, very precocious and productive, with a potential harvest window from June to mid September. Fruit accidentally knocked off ripen decently as early as mid June,but I start picking July 15th. A few fruit hold as late as mid September. Fruit hold well on tree and on the table for a long time without developing off flavors. Later season fruit can be very coconutty. It is our family favorite but is a rich Indian/West Indies flavor which not everyone prefers. Depending on harvest time, it can be very similar to Dwarf Hawaiian, Carrie, Julie,and Graham. It is like having a cocktail tree of those varieties. I think it is better than any of those at its peak. Elite at its best late in the season, but consistently at least very good. If not thinned fruit size will vary from small if in clusters to over a pound. 620 grams was the heaviest I weighed.
I have never tasted 40-26 but at its peak it tastes like Alex's description of that mango.
Fruit and tree seem spotless and free of disease. If I could only have one tree it would even edge out Pickering for the spot because of its extended season and the fact that it is almost like having a natural cocktail tree of mangos in its family tree.