All the examples are correct.
Also, I am inclined to say we prefer compound forms with "самый" in colloquial speech. Yes, I think the synthetic forms with "-ейший/-айший" seem to be more bookish.

Quote Originally Posted by M.Vonok View Post
St. Petersburg is one of the most beautiful cities of the world.

Санкт-Петербург - один из самых красивых городов мира.


How would this sentence which utilizes the Simple Superlative be translated? Would it be translated as - St. Petersburg is one of the highly beautiful cities of the world?


Санкт-Петербург - один из красивейших городов мира.
Sorry, did not notice this question for the first time.

No, it would not be translated that way. Both sentences mean the same:
Санкт-Петербург - один из самых красивых городов мира. = Санкт-Петербург - один из красивейших городов мира. (St. Petersburg is one of the most beautiful cities of the world.)

To be more precise,

the compound form is good for both colloquial and literary styles. And it always has the same meaning: the highest degree of quality.

самый красивый город = the most beautiful city, самый богатый человек - the richest man, самое высокое здание - the tallest building etc.

the synthetic form (formed with "-ейший/-айший" where "-айший" is used after hushes - ш ж ч щ) can have two similar, but slightly different meanings:

1) the same as the compound form with "самый": красивейший город = the most beautiful city, богатейший человек - the richest man, высочайшее здание - the tallest building etc.
This usage, as I told above, is mainly typical for the literary texts. And it is usually supplied with some explicit limitations:
красивейший город в мире, богатейший человек на земле, высочайшее здание в Европе etc. This kind of limitation is not mandatory, but it is usually present. So, you can undoubtfully tell this usage (1) apart the next one (2).

2) a slightly different meaning of this form is figurative: a very high degree of quality (but not necessarily the highest). It is typical for colloquial speech:
красивейший город = a very beautiful city, богатейший человек - a very rich man, высочайшее здание - a very tall building etc.
We often use this kind of metaphora to make the speech colorful. However, in this case limitations are never expressed.

Finally, the compounds with "самый" are not used with meaning (2), they are always understood as meaning (1).

Hope this answers your question now.