As top, you counter top, as mid, you counter mid. as adc, you counter adc, and as support you still counter the adc. who should jungle focus on countering?
You answered your question on your own - what's the last enemy left? :)
Jungler should counter enemy jungler. How to counter? Know the enemy jungler route (for example Amumu must start with Blue) and try to exploit that.
Your aim as a Jungler is to take enemy buffs, and deny other jungler experience and gold from his jungle so that he won't be any threat for your teammates later in the game.
I agree with iber in that you should counter your enemy jungler.
Well unfortunately, you can't counter an enemy jungler just by picking a certain champion that does well against your enemy. It helps mind you, but won't win you the game. For example, a Shaco early game won't be able to 1v1 an Udyr, but a good shaco would be gone before Udyr gets his first hit off.
To add to what iber said, countering your enemy jungler is about taking Blue and Red buffs whenever you can and deny the enemy team the camps in their jungle. Now you can't just walk willy nilly into an opposing jungle and expect to come out on top. Yes, its been done (even I have done it) but the risk usually outweighs the reward. This process actually takes a bit of good timing and map awarenss, when you see your opponent gank top and you just so happen to be clearing bot camps this would be a good time to walk into your opponents bot jungle and clear a camp or two (you might even get lucky with a Blue or Red buff). Don't forget to place a ward while your in their jungle, vision could give you an easy kill, and/or easy escape.
If your really wanting to force a few plays, after a gank bring a lane or two with you into their jungle to surprise them, this is usually best done if you've been a warding fiend. I feel that trying to hard to take your enemies jungle or force a gank is where most junglers fail. You should be quick and decisive, when stealing camps you should be in and out no fooling around hoping to catch the opposing jungler off gaurd. When ganking you should gank then get out as your opponent is already on his way to your camps. This is a good time to catch him off guard.
All in all what you should take from this is that no champion jungler counters another, its just how well you use your champion in the jungle. Even in season 3, Junglers win the game.
I'd like to add a late answer to this question because a lot of things have been left unmentioned.
Don't pick your jungler based solely off their jungler
You're not in lane constantly against the other jungler, so you shouldn't think in terms of countering. Yes, there are times where you should think "Can I 1v1 the enemy jungler" but that shouldn't be your priority.
You should be choosing your jungler on either:
1. If you're good/comfortable jungling with the champ.
(This is pretty self explanatory)
2. How well it fits within the team composition.
You need to ask yourself if your choice of jungler benefits your team or not. Do your lanes not have CC? You might want to grab someone with CC. Do you have a tanky initiator? If not, play someone with initiation. See where I'm going here? I can't stress how important team comp is if you want to win at higher ELO.
Once you're in the game, then you can think about 'counter play'. This is where you can finally think to yourself "Can I beat their jungler 1v1?". If that's the case, you can proceed to counter jungle and try cheeky maneuvers to win the "jungle". But you shouldn't consider this until you've determined what's best for the team. Perhaps your team called for a support oriented jungler like Maokai. Maybe then it's not the best idea to try to force a play on the enemy Xin Zhao's red buff without ample knowledge of his whereabouts.
I know this isn't part of the question but some people have been giving answers regarding which lanes to gank. Personally I'd disagree with the notion of 'helping the weaker lane'. Yes there are times to help get your lane back into the fight. But I think you should be looking to
1. Gank 'attractive' lanes.
Consider your teammates CC and the enemy's escape before just trying a gank.
2. Gank snowball lanes.*
Giving a snowball champion like Riven or Akali a kill or two means you can leave them to dominate their lane and then focus on the weaker lanes. Just trying to gank a losing bot lane that doesn't have CC means you're just going to wasting your time when you could be making a bigger impact on the game elsewhere.
Of course these are just my opinions. You may find other players to jungle a bit more differently.
You should gank whatever lane is having the most trouble while also keeping your own level in line with the team's. The jungler acts as a band-aid, going where ever he is needed. If the enemy jungler is the problem then by all means counter him.
I was arguing with a friend a week ago, some of the players get annoyed when their jungler ganks too much and some of them get frustrated when their jungler does not gank at all. As a jungler I believe that it's your job to keep a balance in the game, help the struggling lane. Some of them just need a little advantage, like a kill. Try not to take the kills from the lane. I have lost games where I get harrased with a Darius playing as Irelia, and when I nearly secured the kill, our jungler comes in and takes the kill. Because I didn't get the gold for the kill I quickly die when our jungler leaves because the enemy jungler comes in and gank me.
So just keep an eye on your lanes if your allies need help, help them. Don't killsteal unless you are sure that your ally can't secure the kill, assist whenever you can and try to give the laner kills. Don't gank just one lane, and keep a good communication with your team.
Contrary to what people on the forum say is that you should NOT always help losing lanes. For example when bot died a few times they will be at lower health then their counterparts most of the time. (especially when they have a longer range over your teammates). If you force a gank on that lane the chances are very high that at least 1 of your teammates will die in the fight resulting in your team requiring 2 kills to get ahead.
That is why it's crucial that when you gank "losing" lanes they need to be healthy because they already are behind. So just ignore everyone who is at 50% and screams "FFS JUNGLER HELP TOP ME 0 GANKS NOW HE IS FEED!!!" at their jungler to come in for a gank. It will probably result into a 1 for 1 trade because your losing lane will most likely die in the encounter. Taken in consideration when the enemy jungler counterganks you will be in an even worse situation. So your wasting your time because of this risky move, instead you could have ganked a healthy lane to snowball them.
I am not saying you should never help your losing lanes, but always be wary that they are healthy enough to receive a gank.
Hope this will help you in your judgements on which lane to gank.