Is it 'rush-hour' or 'peak-hour'?

Either. Generally transport operators in Britain know of two such periods, the morning and evening peaks, and both might be called 'rush hour" informally. The hyphen is optional.

Thanks, centrox.

"Rush hour" is when vehicles might be using the roads the most. The bus useage may or may not be have its "peak hour" at the same time.

Thanks, PUNKEY.

That may be your local usage, but in general they are the same.

One is unlikely to hear "peak hour" in North America. The use of "rush hour" is ubiquitous, and what one will almost always hear on radio and television, which report on traffic conditions every day during rush hour. The term rush hour is also not hyphenated in North America.

What one is more likely to hear is "off-peak hours," at least with regard to retail establishments--stores, clubs, restaurants: "Hey, let's go to Joe's Deli, he always runs specials during off-peak hours."