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That's a difficult question.
LW1 has qualities. It's a bit gritty, a bit brutal, and introduces Riggs with his full gimmick : the temerity due to his death wish. It's what makes his character stand out. in the sequels, as he gets some "family" and sense of belonging, his madness ceases to make sense, it becomes the wacky adventures of the wacky cop for no reason.
But LW1 also has a few boring clichés. The long ritual-like fistfight duel at the end, the "princess" kidnapping, the capture and torture. So, beyond the nice gimmick, the story itself, the sequence of events, is a bit bland.
LW2 starts to lose the point of Riggs, as he gets less reasons to die, and re-ignites it near the end with one of the worst clichés, the "resolution" of the past as everything-was-on-purpose, everything-had-a-meaning. everything-is-tied-together. It ruins the death of Riggs' wife. But on the other hand, it has a little more humor, the story is a bit richer, the South African baddies are original, and there's Patsy Kensit, and there's George Harrison on the best crane zoom out. Also, the toilet scene.
LW3 introduces the Joe Pesci character. It's fun. It gets heavy pretty fast. And the snowballing family values arrives in full force. It becomes a sitcom, and gets worse from there.
So, it depends on the aspects. But I'd say Lethal Weapon 2 gets the best balance and the most highlights.