If you come to Iran with airplane, Absolutely you visit Tehran in first steep, Tehran is very big city with more than 15,000,000 population and have very bad traffic.
Tehran in middle of Iran, but it is closer to north of Iran, distance from Tehran to west or east is same, for example distance between Tehran and Shiraz is 1000 km and to Esfahan is 450 km.
Tehran’s weather is pollution specially in winter, but north of Tehran have good weather in winter and summer.
Tehran don’t have long history the same as Shiraz or another history cities but you can find good museums, shop center, nice place for rest in this city.
Tehran is expensive for Iranian people but for guest people is cheap, Tehran have nice night and i like it more than day. Tehran in Summer is very warm and in winter is so cold because it is near Damavand mountain, tallest mountain in Iran.