A beautiful hot dish can become a problem if the journey is long and the destination has no free oven. Ask what space and equipment will be available before deciding what to bring.
Choose a dish with an appropriate transport plan. Keep perishable food safely chilled or hot as required, and use suitable containers. A salad with dressing separate may travel better in texture than one already tossed, but it still needs temperature control.
Bring a serving utensil, name the dish, and supply an ingredient list when useful. Tell the host if it needs reheating or must remain chilled. Do not arrive with an unannounced task that occupies the whole kitchen.
If the logistics are awkward, bring something simpler. A contribution is most helpful when it fits the gathering, not when it demonstrates the most complicated thing you know how to cook.