Spellcasters are most fun!
Old Wizards (low con, even lower hit points) who have been busy using their spells to make items or perform nasty icky dark rituals are great for low level PCs. You can give them Big Plot elements (powerful wizard, powerful icky magic!), with low level risk. A few spells (suggestion, chill touch, charm person, hold person, gust of wind, even magic jar) can be used to seriously threaten a low level party, without a major risk of a total party kill.
You can do the same thing with priests, but it is much more difficult and dangerous. 2 or 3d8 of touch attack damage is BAD news for the level 1-3 PCs.
The main things to avoid is High Strength monsters, such as Ogres, Giants, Trolls, ettins, Bears, etc. A lucky shot, never mind a crit, and you have one or two dead PCs.
Another tactic I used recently was a small room with a stone golem. The challenge: Loot the room as much as possible, without entering and thus triggering the golem's 'kill intruders' command. The PCs had great fun with it.
