Heh, never actually thought of Relicanth. What set do you use? I might consider him for my team.
Speaking of underrated pokemon, I was using Golurk earlier and even though I was told he was pretty good...I've had no success with him yet.
Slow as shit, so-so defenses, and always seems to fail to OHKO important threats.
Y U FAIL ME GHOST-ROBO?!
For Dragonite specifically, you want Adamant 252/ 252 and a Choice Band. Dragonite / Multiscale is OHKO'd by Head Smash unless it has bulk invested. The best that Dragonite can do is EQ, which only has a 13% chance to OHKO with Choice Band or a DD boost. That's with no investment in bulk on Reli's part.
The moves for that are Head Smash, Waterfall, EQ, and a filler move, Double-edge is okay as another way to exploit Rock Head, I guess.
If you want a little more flexibility, Reli's defense gives him a fairly easy time getting a Rock Polish off, just replace the filler move in the previous set. As for items, a life orb could work, or a lum berry. It's a fairly open slot. Likewise with speed, the previous set can be used with a scarf instead, but it misses the OHKO on MS Dragonite; in return, it can challenge other threats like Tran and Ttar a bit easier.
Always use Rock Head and Head Smash, that's the main advantage Relicanth has. As a side note, if you can successfully use a swords-passing Ninjask or Scizor (if those exist), Relicanth really enjoys the extra power. Don't over estimate its Head Smash; the move wrecks things like flying dragons and fires, but if it's going neutral, physical bulk will hold off Relicanths lack-luster attack stat... You may do a nice chunk, but possibly not enough, and Relicanth's defenses don't worth well against special attacks that are un-resisted, or you know, grass moves.
Edit: The Choice Band is to get through Multiscale; if it's at anything less than full health, it'd take MAJOR defensive bulk to survive an unboosted Head Smash; with the CB it's basically a OHKO if it hits Dnite, and without the CB, it just misses out on the Multiscale OHKO. Regrettably this also means the SubDD set is the one that's going to screw it over. The fairly rare special variant is a pain too, since Reli isn't reli prepared for special hits...