Hard to say for sure, but my favorites in respective order are Jyn the Sniper (can deal the most damage and has the highest mission survivability chance) Nathan the Engineer (can regenerate health) West the General (abilities) The Juggernaut (toughest) Dex the Mercenary (good weapons and explosives)
If you're playing in a larger, wider level, the sniper is by far the best choice. If you're in a small, confined level, go with either the juggernaut or the engineer.