The most powerful weapon in the original Chrono Trigger was the Rainbow Sword, which I've already gotten. However, I've seen suggestions around the net that the DS version has something even better in the Lost Sanctum. I've completed all of the LS quests (annoying as hell though they are), and got the Icewyrm... but the Icewyrm is still less powerful than the Rainbow. Is there something else that I'm missing?
Technically speaking, that'd be the Dreamseeker. Obtained after defeating the Dream Devourer, it boasts an attack power of 240 (to Rainbows 220), and crit rate of 90% (Rainbow's crit chance is 70%). In order to obtain it, you'll need to defeat the Dream Devourer, let the credits roll, and create a new save file. When you start your next New Game+, Chrono will have the weapon.
Practically however, since Attack Power caps at 255, you may find other weapons with different effects to be more desirable once you're at a level of power at which you can acquire the Dreamseeker in the first place.