Software-defined networking (SDN) has introduced flexibility and ease of management in the network. SDN offers centralized and fine-grained control over the web it has become an industry trend and is adopted in various areas of communication and networking. Controllers in SDN play a pivotal role, and their performance is key to the stability and scalability of the network. This study evaluates different SDN controllers i.e., NOX, POX and RYU. The performance of the controllers is analyzed based on Quality of Service (QoS) parameters, including Flow Setup Latency, Round Trip Time (RTT), Initial Ping Delay (IPD), Jitter, Throughput, and TCP & UDP Bandwidth. Different-sized networks are simulated, by varying the number of switches in linear and tree topologies and hosts in a single topology, using Mininet SDN simulator. Our results show that the performance of RYU is very consistent and shows small variation in the performance with increasing network size and traffic loads as compared to NOX and POX.