These matchups are for the initial counter jungle stage in solo queue, often after each champion gets one buff and often ends when the enemy hits level 6 as it becomes extremely dangerous to go in solo to steal monsters at that point. (Not complete as there are obviously many champions that can jungle but I've just put down common ones)
Easy unless he is outrunning you with double buff as he actually deals quite a bit of damage. Extremely vulnerable before double buff and level 4 as his skillset has no burst and almost anything can steal his stuff.
Moderately Easy, it's somewhat hard to hit a rupture on a bloodboiled Nunu with wraith bonus (if he gets a kill) and Chogath runs out of mana just as fast as Nunu
Moderately Easy, Mundo has trouble sustaining his health in the new jungle and if anyone attempts to soften him up with auto attacks or ice blasts, it basically stops him from jungling until he has his laners deal with the invader. Obviously don't try to duel Mundo but rather run circles around him dealing damage if he ever tries to go near a camp as Mundo loses a huge amount of HP when jungling.
Hard, she is fairly mobile thanks to her skillset, she does big damage and is almost unaffected by Ice Blast. Also possesses huge burst damage spells to make stealing camps from her harder. I'd probably avoid counter jungling her unless you know where she is on the map as she will probably hurt Nunu very hard.
Hard, it's usually a waste of time as you can't see Evelynn unless she is actually doing a camp or runs into you, the normal brush camp spots may not be useful to scout her jungle in as she has to get close for you to realize she is in the area. It's probably better to just counter gank her or take her stuff when she does gank than just straight up run at her jungle.
Moderate difficulty, for most invaders he is a easy champion to counter jungle but since Nunu doesn't deal enough burst damage before Fiddle can wind up another life drain, often it'll come down to a skill matchup. However some advantages Nunu has is that he knows when hes around due to his passive and he has burst damage on monsters so he can steal them from right under him if he has no smite.
Moderately Easy, Hecarim has very little chance to do much to Nunu after his charge is down, and Nunu's Ice blast actually basically removes Hecarim's passive AD as it cuts all of his bonus movespeed down. Hecarim also lacks any burst damage on camps so consume is able to take anything.
Fairly hard, these guys all are monsters with double buffs so if you can't prevent these champions from getting red, you'll probably be cced or damaged hard enough for the enemy laners to intervene and finish you off. I advise against counterjungling these guys UNLESS you have vision of them.
Somewhat easy, these guys mostly just CC Nunu down for allies to come help as they're unable to deal with the sustain Nunu does and his burst damage on consume.