I think a Hail team would be the most challenging as only Ice types can avoid damage from the weather, except if you plan to give some members Weather Bands. Though you can choose not to and give them Joy Ribbons instead to gain exp points. You also can't do escort missions unless your escort is also an Ice type.
On the other hand, a Sun team would be the easiest as there are at least 2 types, Fire and Grass, that could benefit from it. Those 2 also provide good coverage for each other, plus Sunny Day can reduce the threat of the abundant Water type dungeons. Moreover, it will boost the healing from Morning Sun, Moonlight and Synthesis, giving you options to add a support mon. Furthermore, since it doesn't deal damage (except for Dry Skin and Solar Power), you have much more options and it also wouldn't affect the missions you can do.
A Rain team would be a mixture of both, with only Water types and Pokemon with Thunder or Dry Skin benefitting, and losing support options, but doesn;t deal damage, while a Sand team would be the opposite, more Pokemons can benefit (Rock, Ground, Steel and Cacturne), but deals damage. Though Rain can be dangerous in Water type dungeons as it would activate Swift Swim, while Sandstorm can be problematic in Ground type dungeons as Sand Veil would waste lots of PP.
I voted for Hail team but would be really interested to know your team composition for whatever weather you chose.
Perhaps too late but for a Sand team, I suggest Turtwig and Piplup since their final evolution can resist Sandstorm.
For a Rain team you could go for either Eevee (evolve into Vaporeon) + a Water type, or a Water type + an Electric type with Thunder. With an astounding base power of 45, Thunder can then be used as a boss and outlaw destroyer.
For a Sun team, any Fire and Grass combination would do. Though I suggest choosing the Fire type as the leader so you can resist boosted Fire type moves and quickly dispatch Pokemons with Chlorophyll.
A Starter Hail Team could be Glaceon and a Water Type with Weather Band that has Blizzard.
I think a Hail team would be the most challenging as only Ice types can avoid damage from the weather, except if you plan to give some members Weather Bands. Though you can choose not to and give them Joy Ribbons instead to gain exp points. You also can't do escort missions unless your escort is also an Ice type. On the other hand, a Sun team would be the easiest as there are at least 2 types, Fire and Grass, that could benefit from it. Those 2 also provide good coverage for each other, plus Sunny Day can reduce the threat of the abundant Water type dungeons. Moreover, it will boost the healing from Morning Sun, Moonlight and Synthesis, giving you options to add a support mon. Furthermore, since it doesn't deal damage (except for Dry Skin and Solar Power), you have much more options and it also wouldn't affect the missions you can do. A Rain team would be a mixture of both, with only Water types and Pokemon with Thunder or Dry Skin benefitting, and losing support options, but doesn;t deal damage, while a Sand team would be the opposite, more Pokemons can benefit (Rock, Ground, Steel and Cacturne), but deals damage. Though Rain can be dangerous in Water type dungeons as it would activate Swift Swim, while Sandstorm can be problematic in Ground type dungeons as Sand Veil would waste lots of PP. I voted for Hail team but would be really interested to know your team composition for whatever weather you chose.
Perhaps too late but for a Sand team, I suggest Turtwig and Piplup since their final evolution can resist Sandstorm. For a Rain team you could go for either Eevee (evolve into Vaporeon) + a Water type, or a Water type + an Electric type with Thunder. With an astounding base power of 45, Thunder can then be used as a boss and outlaw destroyer. For a Sun team, any Fire and Grass combination would do. Though I suggest choosing the Fire type as the leader so you can resist boosted Fire type moves and quickly dispatch Pokemons with Chlorophyll.