Gyara is only going to be dancing in the endgame, though (In Kanto, no less). On the other hand, Lappy is going to be STAB Ice Beaming the E4 while Gyarados is nuking with Aqua Tail/Ice Fang.
Frankly, as a person, I hate Gyarados. However, I do know it's a really damn good Pokemon ever since the Physical/Special Split happened. Gyarados also happens to be on the Slow Exp track, meaning it takes so much longer to level it up than the others (See my GSC tier list for more info on that, but it really is significant). On the other hand, Lapras is also on the Slow Experience path. Pick your poison, insta Level 30, or Level 15?