"What is the oldest region?"

Its hard to say, mostly because of how things were not well documented during the Foundation Era, and that 'regions' are a relatively new thing. I would say either Shadowfoxia, Meadowen, or Seglexas. The first region established during the Imperial Federation was Meadowen.

"What is the most populated region?"

Definitely Meadowen.

"What region has the most cities?"

Meadowen has the most with four cities. But including colonial settlements, it would be Vineland with five total cities.

• Seglaeghn - 0.0104  (sq mi)

• Shadowfoxia - 0.0135 (sq mi)

• Meadowen - 0.0145 (sq mi)

• Stallion - 0.0190 (sq mi)

• Hawth-Koufax - 0.0400 (sq mi)

• Metbir - 0.0457 (sq mi)

• Vineland - 2.8500 (sq mi)

• Meadowen (5)

• Metbir (2)

• Hawth-Koufax (2)

• Stallion (1)

• Shadowfoxia (1)

• Seglexas (1)

• Vineland (0)

• Meadowen (58%)

• Stallion (17%)

• Seglexas (13.2%)

• Metbir (5%)

• Hawth-Koufax (2.4%)

All other regions are 0%. This is evaluated by the percentage of marks owned by citizens from the region compared to total marks owned by citizens.