I love many things in Paris: the bakeries, crepe stands, a wide range of restaurants, and many other things that are enjoyed by thousands of tourists and locals every day.
A few very specific favorites: Dehillerin and A. Simon for kitchen supplies, Catherine's for perfume, Bouchara for fabric and Monoprix for inexpensive gifts, souvenirs and modestly priced toiletries.
I love Angelina's for Chocolate Chaud, and Le Souffle for the best Grand Marnier Souffle I ever had, Le Train Bleu for a unique, turn of the century setting, and the Rodin and the Marmottan Museums for their very "doable" collections that can be seen and absorbed in a couple of hours, yet enjoyed in more depth on return visits.

I find it impossible to define favourite things in Paris with one exception which, alas, is no more. That was the willow tree at the end of Sq.Verte-Galant, Pont Neuf. But, there are things that happen which stick in your mind such as.......walking up Rue Mouffetard and stopping in a doorway for several minutes to listen to a wrinkley-faced old man churning out wonderful French tunes on his Frou-Frou(barrel organ). The scene.... the music........ah, viva Paris!!

Arc De Triomphe,

It is the obvious tourist attraction, but I believe its impact all depends on what time of day you go. I always aim to get there just before dusk. It is magical to watch Paris light up. From La Defence to the obvious La Tour Eiffel. Sends one of those chills down your spine.

Paris is a walking city, so much of its appeal is not sights, but taking in the surroundings. Personally i found Rome rather overrated. When you are in florence have a day trip to Sienna.