Periodic table groups - which grouping is "right"? In searching online, I've noticed there are a lot of different ways to group the elements of the periodic table.

Take mercury in the two tables linked below, for example:


http://i.telegraph.co.uk/multimedia/archive/02101/periodic_2101916b.jpg
http://www.sciencegeek.net/tables/PToE_basic.pdf


On the first table, it's explicitly outside of the transition metals. On the second table, it's included in the "transition metals" group.

Mercury is an example of this inconsistent classification, but in general, why is there no canonical grouping for all elements, across the periodic table? I see some tables mentioning "metalloids," but others not doing so. Or, sometimes elements are classified in entirely different groups altogether (like zinc, cadmium and mercury in the above-linked tables). Or, the lanthanides and actinides being included sometimes in the transition metals group, and sometimes not.

So which classification is right? Or rather, which one is the "most right"? Really: which one should I focus my efforts on remembering?
There is no "most right classification scheme" for the elements of the periodic table, worth memorizing above all others.  This is because there is no single, 'universal' set of criteria that effectively, unarguably, and neatly divides the entire periodic table into groups. Depending on the properties of interest, different classifications/groupings are more or less appropriate.