Willymy,

I have stayed in the Istanbul Hilton Bosporus, which is a bit off the main old town, but has great vistas over the Bosporus straight. Well worth IMHO.

Paris probably best spent at the Hilton Arc de Triomphe, but may be difficult to get as a standard reward.
Alternatively, if you plan to see Versailles Castle, you may look into staying at the Trianon Palace (WA), which has a superb locatoin next to the Versailles gardends.

In London I like the Hilton Green Park, which is in Mayfair and on the smaller end.

Cannot comment on Rome and Athens
Happy New Year!
DAvid