Background: Extensive-stage small cell lung cancer (ES-SCLC) carries a poor prognosis, with fewer than 7% of patients surviving for five years. While immune checkpoint inhibitors (ICIs) combined with platinum–etoposide have reshaped first-line treatment, no head-to-head trials exist comparing available regimens, leaving the optimal therapeutic choice undefined. Methods: A systematic literature search identified phase III randomized controlled trials (RCTs) evaluating first-line ICI-based regimens in ES-SCLC. Individual patient data (IPD) were reconstructed from Kaplan–Meier curves using the IPDfromKM algorithm. Indirect treatment comparisons were performed using Cox proportional hazards models, with chemotherapy alone as the common comparator. The indirect comparison of tiragolumab efficacy was unanchored due to the design of the SKYSCRAPER-02 study. Restricted mean survival time (RMST), truncated at 27 months, was calculated as an additional measure of treatment effect. Results: Six RCTs were included. All ICI-based regimens improved overall survival (OS) compared to chemotherapy alone, except ipilimumab (HR 0.97, 95% CI 0.86–1.09). Serplulimab demonstrated the most favorable OS benefit (HR 0.55, 95% CI 0.48–0.64; RMST 16.73 months), representing a gain of approximately 4 months over chemotherapy alone and 1.8–2.3 months over atezolizumab and durvalumab. The addition of tiragolumab to atezolizumab plus chemotherapy yielded no significant advantage over atezolizumab alone. Conclusions: This IPD-based indirect comparison suggests that serplulimab plus chemotherapy may offer the most favorable OS estimates S benefit among first-line ICI regimens for ES-SCLC, while durvalumab and atezolizumab remain effective standards of care. These findings are hypothesis-generating and highlight the need for prospective head-to-head comparative studies.