Yes, the Dart Gun is fantastic for Deathclaws. In fact, I pretty much don't use it for anything else. Except griefing Raiders when I'm in a bad mood. Haha, it's so fun to make such savages, who had the arrogance to attack a level 20 hero with more guns in his clothes than teeth in his mouth, run away - SLOWLY - in disgrace.
Seriously, I would say that the Deathclaws present the most serious singular threat in the game. I had no problem with the Super Mutant Behemoths, but then I always understood that Small Guns aren't going to take it - use grenades (and not in VATS either) and other Explosive weapons.
But really, it comes down to your level. If you're level 20 and have no DLC, then nothing will really top you. That is, unless you've completely messed up your stat division, which is unfortunately very easy to do. Meanwhile, being level 5 you can have the bad luck of unknowingly coming across the random event Deathclaw, or travelling too far north to their Sanctuary and saving at the perfectly wrong time.
Personally, I had a big hatred going for the robotic enemies. Nothing will touch them really except for high-end, expensive weapons like the Fat Man or specially designed stuff like Pulse Grenades. I find that horribly cheap, for all its realism. Of course, ninety-nine times out of one hundred, I will have Pulse Grenades on me.