Gah, this is hard. I don't really have much trouble with Beldum, thanks to my somewhat high-level Venusaur, which has Sleep Powder.
Probably Tauros, because I've only ever found it in the Kanto Safari Zone.
Spiritomb. There's just too much junk to do before you can unlock the possibility of catching him.
I have one. We should trade.-Beldum, nuff said. Capture rate of 3 for a non-legendary? What a pain.
-I have to agree with the OP that Larvitar is hard to find and catch. In my Diamond, I used PokeRadar for a long time in Route 207, but I still could not find a single Larvitar!
-Rare Pokemon in Safari Zones. In my Diamond, I had encountered Kangaskhan for few times, but I always failed to catch it. Even finding a Shroomish in Great Marsh is a pain either; it took me a very long time to find and catch it.