I'm split between 1 and 2. 1 is definitely more of a classic, has some of the best dialogue of the whole series, is set at Christmas so it can join Die Hard and Gremlins in the "it is a Christmas movie or not" debate, and has Gary Busey playing himself an unhinged psychopath. 2 feels more fun since you get the Riggs and Murtaugh chemistry out the gate and has the better set pieces (toilet bomb, that cool office fish tank, house on stilts, cargo ship with containers full of cash, etc).
Guess it depends on what side of the bed you woke up on