`learning_rate`: shrinkage (0.01-0.3) - `max_depth`: tree depth (3-10) - `n_estimators`: boosting rounds (100-2000, with early stopping) - `subsample`: row sampling (0.5-1.0) - `colsample_bytree`: column sampling (0.5-1.0) - `min_child_weight`: minimum child weight (1-20) - `reg_alpha`, `reg_lambda`