Which aircraft has the largest seating capacity?

The A321 NEO is designed with a larger seating capacity compared to the other options listed. This aircraft incorporates advanced aerodynamics, new engine options, and an improved cabin configuration which allows for more seats while maintaining comfort. The NEO (New Engine Option) version features higher efficiency engines and often has a dual-class or all-economy layout that can accommodate up to 240 passengers in maximum-density configurations, making it suitable for high-demand routes.

The other aircraft, while capable and popular in their own right, generally offer fewer seats. The A320 typically seats around 140 to 170 passengers, the A320 Spaceflex is an optimized variant of the A320 with some capacity improvements, but it typically doesn't match the A321's range. The A321, while having a larger capacity than the base A320, does not reach the maximum capacities that the A321 NEO can handle. Thus, in terms of maximum seating capability, the A321 NEO stands out as the largest among the options listed.