#1.Nidalee because of the spear throw, healing, traps, transform, jump attack. Good midgame to late game. Easy to run in and fight and easy to jump out and escape.
(Weakness lower than average magic armor)
#2.Shaco because of the jack in the box free farm at level 1, cloak crits, poison shivs, 25% dmg passive from behind and clone ultimate. Clones share your passive and item abilities. Easy to get first blood with this character with a good lane partner. Good late game as well and can use the clone to scout and kill without risk.
(Weakness fairly easy to kill midgame, if fighting against a good team that wards)
#3.Katarina if you want to reduce healing with the bouncing blade+killer instinct with ability to teleport to the enemy good AP ultimate, which can easy kill low magic armor champions KI+S+DL easy kill.
Weakness low hp (easy to get killed)
#4.Warwick One of the best junglers, good for pugs, fantastic passive allowing you to level up fairly quickly and let your lane partner level. A decent hero killer if the hero isnt a tank. Good support on physical dps heavy team since hunters call applies to all allied heroes. One of the weaker late games.
(Weakness ultimate nerfed, less time though it added 30% life steal.)
For dps go with Shaco free level 1 golem killer or Warwick Mr. Jungle
For AP go with Katarina though you may try dps
For ranged support go Nidalee
Personally out of the few times I tried the heroes I prefer Nidalee and Shaco.
To each person their own though. You should wait until you get several responses though.
Also pick depending on your own personal skill level and what kind of play style you use.
I am also a semi-casual player so wait until you have at least 5 responses. The top is my personal opinion based on personal usage and playing against players who have used them.
If you havent done so already you could always buy every hero that is 450.