The harder enemies are the ones that give a buff; as stated, the Ancient Golem and Elder Lizard. You can tell in-game they are harder because they have the buff effect on the unit (swirling runes going in a circle around it), but sadly this isn't displayed on the minimap.
Originally Posted by criptix
imho i would say ghost > tri wolfs, coz the ghost are 1 big + 3 little creeps with a lifeleech aura
I mostly agree with this, It really depends on your champion. AoEs utterly destroy the Wraiths. But the Wolves are definitely the weakest.
Baron Nashor respawns after 8 minutes.
Dragon respawns after 5 minutes.
Elder Lizard and Ancient Golem (groups) respawn after ~5 minutes, I believe.
The smaller groups (Golem Pairs or Wolves) respawn after about two minutes.
If you kill some of the group but not the entire group, the missing units won't ever respawn. This is sometimes used tactically: Go get the buff from the ENEMY's Ancient Golem, leave small ones alive so it won't respawn. The idea is that the enemy won't have easy access to these buffs.
High level play often revolves around the buff-giving neutrals, Dragon, and Baron Nashor. They are extremely important. It's also "safer" to be jungling, especially as a group, as the enemy doesn't know where you are.