That is such a hard question! @.@ Hmmm, well I don't know about 'best' but one of my favorites is Rare - Or Rareware as its sometimes called.

As a child I was a fan of lots of their products; Donkey Kong Country, Banjo Kazooie, etc. Of course this was all before 2002 - where they were acquired by Microsoft - so I've only played their older stuff.