Explosive Clap is a nice, basic special attack, but it really doesn't do much damage unless he is in the middle of a group of enemies. The cooldown is 25 seconds, so he doesn't use it that often. Definitely worth the point since it doesn't have a major downside like some of his other attacks and is still solid against a single target, so it doesn't get wasted.
By itself, Robot Rampage is pretty good, but not that amazing. It's still a pretty big damage buff, but Make It Sparkle is probably better even just with normal attacks due to the cooldown. However, the combination of Robot Rampage and Make It Sparkle send both through the roof, as long as you use the correct element (slag is never the correct element).
With the correct element and slagging the enemy before hand, you have an attack doing at least 80 times more damage than Deathtrap's base damage that is usable every 13 seconds. The only real downside is Deathtrap can't use it against enemies he can't melee, so it isn't good for farming certain enemies. You also have to be careful not to accidentally slag him or his damage will plummet, even then, it is still quite a lot, but not compared to using it properly because you lose the elemental damage and slag bonus multipliers, which are huge, especially when you lose them 10 times on something that is naturally double base damage.