Accurate forecasting of daily solar radiation is essential for renewable energy management, optimization of photovoltaic systems, and power-grid stability. This study presents an evaluation of six forecasting models: SARIMA, LSTM, NARX, Random Forest, XGBoost, and a stacking ensemble. Daily solar radiation and meteorological data were retrieved from the NASA Prediction of Worldwide Energy Resources (NASA POWER) platform for Poruba, Ostrava, Czech Republic. A chronological data-partitioning strategy was adopted, with data from 2001–2020 used for training, 2023 for validation, and 2024–2025 reserved for prospective testing. The modeling framework incorporated meteorological variables, cyclic calendar encodings, lagged radiation observations, and rolling statistical features to capture short-term fluctuations and long-term seasonal dynamics. Among the individual models, XGBoost achieved the strongest test performance (R² = 0.7704), followed by Random Forest (R² = 0.7565), NARX (R² = 0.7524), and LSTM. SARIMA exhibited the weakest predictive accuracy (R² = 0.7265), reflecting its limitations in modeling nonlinear temporal dependencies. The stacking ensemble, combining Linear Regression, Random Forest, and K-Nearest Neighbors as base learners and Ridge regression as the meta-learner, achieved the highest performance (R² = 0.9997), outperforming all individual models on the prospective test set. Statistical significance testing using the Friedman omnibus test, Wilcoxon signed-rank test, and Diebold–Mariano test with Holm–Bonferroni correction confirmed that the ensemble’s superiority was statistically robust across all pairwise comparisons (p < 0.001). These findings demonstrate that ensemble learning, supported by structured feature engineering and temporally consistent model evaluation, provides an effective and generalizable framework for long-term solar radiation forecasting.