The Power Efficient-Gathering in Sensor Information Systems (PEGASIS) protocol is one of the classical data gathering protocols for wireless sensor networks. PEGASIS works by forming a chain of the sensor nodes starting from the node farthest away to the sink. Data from etiher end of the chain gathers towards the leader node, selected for each round of data gathering, through a hop-by-hop transfer and aggregation process. The leader node transmits the aggregated data to the sink node. In this paper, we investigate the impact of the following leader node selection strategies for every round: Random (randomly selected node), Shuffle (a node is selected as leader only once in N rounds in a network of N nodes), High-energy (node with the highest energy), 2-block and 4-block (the network is divided into 2 or 4 blocks and the leader node is the highest energy node in the randomly chosen block of a round). We study the PEGASIS protocol for both TDMA and CDMA systems. For each combination of network topology (square, circular and rectangular) and sink location (center, origin and outside the network field), we identify the leader selection strategy that yields the longest network lifetime (up to 5% node failures) and the minimum energy*delay per round.