The Cafe de Soleil in Petit Saconnex (bus 3) is very popular, especially among the younger crowd, but...I personally have other favorites:
Cafe de Commerce, Petit Saconnex (next door to the Soleil)
Cafe du Coin, rue Monthoux/rhe de Fribourg (Paquis District, nr rail stration)
Cafe du Petit Chalet, rue de Berne (nr station)
Cafe Vaudois, rue des Alpes (nr station)
and many more.
I suggest that you don't pay more than CHF 20/pers for a standard (moitie/moitie cheese mix Gruyere/Vacherin Fribourg) fondue, maybe 22.00 with Bolets mushrooms. Geneva white wine should not be more than CHF 3 per dl (or 30 per liter). If you pay more, don't expect it to be better.
Favorite Dish The quality of Fondues varies a lot from one place to another. The right mix of cheese plus the type of wine used for cooking is most important. If you want to make your own, it is safest to buy a ready-made Fondue, the best is made by Gerber (available at supermarkets).
The wine that accompanies fondue should be dry (I personally like it fruity dry), but you can also drink black tea instead.
A small glass of (Swiss) Kirsch may be stirred into the fondue or just to accompany the meal.
The bread ideally should be of the previous day, slightly dry, so it is not lumpy after soaking/stirring it into the fondue pan.
The ambience here was like in a little pub, tables all close together so that there was a feeling of just one big group all together. As everywhere in Geneva it was so international there, people from everywhere around.
Favorite Dish We were told that this is the best Fondue place in Geneva and we haven´t been dissapointed.! The cheese Fondue was just GREAT!! We had white and rosé wine with it.
Cafe du Soleil is charming little restaurant where you can eat one of the best typical swiss dishes: the cheese fondue.
It also has a nice terrace during the Summer months to enjoy lunch, dinner or simply a drink.
Favorite Dish I would recommend their specialty: the fondue, cheese that is. Indeed, unlike the fondues you taste in the States that are pure American inventions, the authentic one consists of melted cheese where you dip bread. These can also be mixed with tomatoes and mushrooms.
The fondue bourguignone on the other hand, is a warm stock where you cook your meet and is served with a variety of sauces.
The plat du jour (menu of the day) is always a good bet as well.