Italian, definitely, for speed. Other than that, it's down to what you like. Listen to the best players and check out what they have (I went for Pigini because two of my accordion heroes, Martin Green and Karen Tweed, play that). The player's style is usually the biggest factor, and it will dominate whatever make their instrument, but it still helps you hear the instrument at its best.
Below is a link to some mighty fine ITM playing by Karen Tweed on a Pigini 72 bass 3-voice with handmade reeds, and Colette O'Leary on a battered old 60-bass Paolo Soprani.
http://youtu.be/O_5g5XAkKTg