Want the Pokemon with the highest possible HP for annoying people. I heard that it can have over 700 max HP.
Yes. And that's very useful, considering that it has the lowest defense of all fully-evolved Pokemon.
Blissey's HP maxes out at 714, when it's level 100 and has 31 HP IVs and 252 HP EVs.
However, if you want to annoy people, then you may want something with better defense stats like Chansey (with eviolite) or Lugia.