We stayed in Hotel du Lac in Varenna in May of this year. It was a fantastic experience. Their hostess at the desk, Tatianna, was the best we encountered in our 18 day trip in Italy and Switzerland. The hotel also has a terrace for dining, and the best breakfast selection. We paid extra to have a balcony off our room, and the view was amazing. We went to Bellagio and Mennagio on the ferry and felt both paled in comparison to the beauty and charm of Varenna. We took the local train from Milano Centrale to Varennna-Esino stop, and it was a lovely hour trip. The train station in Varenna has no ticket office, but you can purchase a return ticket (good within a 4 day time frame) when you buy your ticket to this lovely place.
If you choose Bellagio or Mennagio, you still have to take the train to Varenna the get to the ferry dock and ferry to these villages. On your return, you have to ferry back to Varenna to get to the train station.
We took the bus from Bellagio to Como San Giovanni Train station. What we saw of the town of Como also did not compare to Varenna as it was larger and not as quaint. The mid lake region of Lake Como is what we enjoyed.