I have a lapras with its first three moves being psychic, ice beam and body slam. Which move is better for it's last one? Surf or Perish Song? Please note, this is not a question on what is the best move in existence for lapras, or my particular one stat wise, just which one out of these two moves is the better one in general.
I say Perish Song. Since Pokemon in game rarely ever switch, it is a great move alongside Body Slam. Just use Perish Song and then teach Lapras for Ice Beam. This allows you to stall out the opponent and gives you time to do the well needed Healing Up of other team members. It is what I used when I played HGSS and it worked great. I always had it as Ny lead. And use it against Lance and all. Their main threats will just go down really easily.
I would pick Surf; it's more useful in-game (lets you get around on water), Lapras gets STAB from Surf and it also gives some good coverage.
Surf would be good because perish song is too easy to avoid. All you have to do is switch out a Pokemon. Plus surf would also help to get across on water.