The Eiffel Tower	which	has become	a symbol of France	was built in the 19th century as a temporary structure	
The Pyramids of Egypt	which	were built in ancient times	are situated just outside	the city of Cairo	
Mount Everest	which	lies on the borders of Nepal and China	is the tallest mountain	in the world	

Answers

The Eiffel Tower, which has become	a symbol of France, was built in the 19th century as a temporary structure.
The Eiffel Tower, which was built in the 19th century as a temporary structure, has become a symbol of France.

The Pyramids of Egypt, which were built in ancient times, are situated just outside the city of Cairo.
The Pyramids of Egypt, which are situated just outside the city of Cairo, were built in ancient times.

Mount Everest, which	lies on the borders of Nepal and China, is the tallest mountain	in the world
Mount Everest, which is the tallest mountain	in the world, lies on the borders of Nepal and China	

Which one is best?

This depends on the main subject of the paragraph. In the first example if the paragraph is about the history of the Eiffel Tower, then the first sentence is better:

The Eiffel Tower, which has become	a symbol of France, was built in the 19th century as a temporary structure. It was designed as the entrance arch to the 1889 Word’s Fair in Paris…

If the paragraph is about the Eiffel Tower as a symbol of France, then the second is appropriate:

The Eiffel Tower, which was built in the 19th century as a temporary structure, has become a symbol of France. Its distinctive shape is instantly recognizable around the world and is used to represent France on items ranging from tourist souvenir mugs and T-shirts to company letterheads…