If the X - Men were in the Frozen universe, they would bring a new dynamic. Wolverine's healing factor would make him a valuable asset in the cold and dangerous environment. He could withstand extreme cold and any injuries that might come from the ice. Storm could collaborate with Elsa to create more elaborate weather patterns. The X - Men's mutant status could also cause some interesting reactions among the people of Arendelle, who are not used to such extraordinary abilities. It could lead to a story about acceptance and cooperation between two very different groups.
In my opinion, X Factor wasn't the best. There were other X-Men comics that had more compelling storylines and character developments during that time.
It's hard to say for sure. X-Factor had its moments, but there were other X-Men comics in the 90's that were also highly regarded. It really comes down to what aspects of the comics you value the most.
An annual in comics is a special issue that comes out once a year, often featuring longer stories or special events related to the ongoing comic series.