Platinum holds the title of the greatest Pokemon game in my eyes. It offers an engaging plot, an expansive region, the greatest regional PokeDex yet with an incredible amount of variety, memorable characters, unlimited replay value, and just so much more. It's the absolute pinnacle of the Pokemon series. Emerald isn't too far behind from Platinum--and it's mainly the lack of the additions the 4th Generation introduced that hold it back, so it's not exactly too fair a comparison.
HGSS, on the other hand, featured a few cool features like the Pokeathelon and the ability for Pokemon to follow you everywhere, but there were far too many flaws with the game--such as its level curve, lack of plot, uninteresting and boring locales, and monotonous characters--that it couldn't possibly be considered the best Pokemon game, at least in my opinion.
Black and White, for what it brings to the table, it takes significantly more away. A new region, 156 Pokemon, and much-improved graphics were given at the expense of re-playability and features: Black and White don't feature a Battle Frontier, an Underground, a PokeRadar, the ability to grow berries, following Pokemon, Pokeathelon/Contests, or rematches.
It's hard to regard Black and White as the best Pokemon games ever considering this--it all depends on whether you think what was lost, was worth for what we got. I personally don't, and I'm waiting for the third version to really do the fifth generation and Unova proud.
