I have played some 200 + dominion games. (love it)
The top 10 Champs IMO
10) Vayne and Yorick are tied. Both are very strong characters if played correctly.
9)Gangplank - suffers from the lack of status effects but ultimate can be fired anywhere and can deal very good damage.
8)Shaco - solid defender with lot of tricks. can hold a tower by himself even when out numbered.
7) Kassadin - almost impossible to kill in the right hands.
6) Ryze - great attacking champion able to deal a lot of damage in a group, can suffer alone.
5) Akali - great movement and Damage and Twilight Shroud makes tower defending a breeze.
4)Pantheon - the ultimate defender. lots of damage, hit hp and armor and an ult that allows almost full screen movement can single handy sway the tide of battle.
3) Twitch - amazing ganker in dominion. Invis with poison power can damage a large group and escapes very effortlessly.
2)Rammus - tough to take down, ult is a ***** and has great movement around the field A+ dominion Champ.
1)Blitzcrank - I know I am going to get some heat for this but he has all the tools needed to dominate in dominion. Rocket Grab- breaks up formations, dragging opponents into a tower for a quick gank is just too fun. Overdrive also for easy of map movement to defend and attack un protected points. Lots of life makes him hard to put down and aoe Static Field can turn the tide when other champs are trying to launch attack ext.
now other character can be very effective. I myself use Fiddlesticks the most. i love the mass silence and terrify. as it allows me to juggle and crow storm always me to attack towers with abomb! and drain keeps you from having to portal back all the time. but the list is the champs i see the most and are the most annoy and easy of use in dominion
hope this helps.