Ahri is a strong pick really. You can either pick something really deffensive and just farm (if that suits your placestyle) like Galio or Karthus, or you pick something that builds tanky (since ahri has no real sustained dmg), like you said Morde, Ryze, Galio.
I know from experience that Kennen and Vlad win against Ahri, cause Ahri is really weak on the first few lvls (1-3) and Vlad can outsustain her (even after these first lvls) while Kennen can outdmg her (due to his superior autoattacks), so try these out.
Or if you really have problems against Ahri, you can try TF. He is somewhat risky, since she can easily burst you if she gets ahead, but the good thing about tf is: you can just push till lvl 6, gank sidelanes, go back to mid and push on long range with your Qs.
Edit: against a good ahri, morde will loose without the right runes. Ahri will just use AD runes and autoshot you down on lvl 1 or 2, and after the first lvls, she will ask her jungler for ganks, and there is no way a morde can escape from an ahri charm + jungler presence. So only pick Morde if the enemy has a weak early jungler!!
Let's make some assumptions about jungle Amumu:
- requires blue for an efficient jungle start
- can gank at lvl 2
- builds defensive and AP items
- deals primarily magic damage
The first option (as you mentioned) is to steal Amumu's blue. Amumu relies heavily on tantrum to clear his jungle quickly, but this can quickly consume his mana. This prevents him from ganking and drastically slows his early jungle. (note: With jungle camps now providing health/mana, Amumu can still manage the jungle without blue. In times long past, stealing his blue completely destroyed his first jungle clear).
The second option is to ward/counter-gank. Bandage toss gives Amumu a strong lvl 2 gank. With map awareness and a key ward, the opposing jungler can counter-gank Amumu early before he has any defensive items.
Some junglers also have strong skills when dueling Amumu. Mundo, Shyvanna, and Lee Sin will typically win duels without outside help, and these champions excel at counter-jungling him as well. This can be especially true since Amumu tends to jungle with low health early on.
As the game progresses, Amumu's ganks will rely on hitting with bandage toss. Keeping wards in common gank locations and using minions for cover minimize his chances to succeed. Purchasing MR can help, and having cleanse or a QSS help immensely with escapes. Amumu is also susceptible to silences/stuns, so a team with high CC can prevent these. Also, a Banshee's Veil is very effective vs Amumu.
Janna deserves a special mention, as her ult will directly counter Amumu's if he doesn't kill her first.
tl;dr: Amumu is a tanky jungler and great initiator. He can be shutdown the same as other junglers via counter-jungling and early invades. His strength comes through mid to late game, but if he doesn't succeed early, he'll never get there. Strong early-game junglers who constantly steal his jungle are the best counter to him.
Best Answer
Akali has huge advantages in her skill set that allow her to excel in Dominion.
Dominion involves a lot of 1v1 fights
Akali is one of the best 1v1 champions in the game. Her mark of the assassin just gives her enormous burst, even at early levels.
She has an amazing escape mechanism using Twilight shroud
This is perfect for juking in the middle of the map, and perfect for protecting her when she is defending a turret.
Her mobility using via Shadow Dance
You are never going to escape from her, especially if she has a slow. She can also use it to bink to escape by targeting minions. This, along with her Twilight shroud make her a very tricky champion to kill and a champion almost impossible to escape from.
Her damage is hybrid
Is is pretty hard to counter Akali because of her different damage types. Getting the balance of defense, mobility and offensive items is hard enough.
She has innate spell vamp through her passive Twin Disciplines
Her spell vamp combined with the high burst makes her very hard to kill quickly. This also gives her a lot of sustainability through leeching health from minions. Without the ability to nuke her down easily, she can try to escape using her shroud/ult/summoner spells.
More champions play 'glass cannon' builds than on Summoners Rift
Glass cannon champions tend to be easy prey for Akali. A lot of builds are base around kiting and mobility. Kiting just doesn't work on her because of her ultimate and she has HUGE burst which will bring down any squishy champions pretty rapidly.
The Dominion is map is designed to be played offensively
Akali can gank pretty freely as there aren't really any 'safe zones' you can run to, i.e. behind your tower line on normal matches. This leads up to her snowballing pretty quickly as she is just fantastic at picking off roaming enemies.
She snowballs rapidly
Her burst gets insane if she is fed, combine this with the inability to escape from her and your team are in for some big trouble.
Akali is based on energy
She is not the only manaless champ, however manaless champs have always had the advantage of being able to spam skills at no cost. This means Akali can constantly push lanes and leech health through her passive, without having to worry too much about going back to base to heal up.