A market order is an order to buy or sell a stock at the best available price, which can typically be executed at the time of the order.



To sum up:
Market order has the best chance of filling, but the filling price is not certain.
If you’re trading large-cap or highly liquid securities, placing a market order usually enables immediate order filling at a close price. If you want to control your filling price, or you’re trading small-cap or illiquid securities, a market order may not be suitable.