Well, after few really good wins with this build I have decided that I want see what u guys have to say about the build for Sion in Dominion.
This build is the way I see and think that Sion should be while playing with him.
shortly, I based the Attack Damage at the ENRAGE at first and at Items in the end.
the Items at first are so no 1 could run from u and every Basic Attack will be strong.

after playing about 15-20 games and killing about 10-20 champs die'n 5-7 times and getting 1-2 place each game I can say that for me its an excellent build,
hope you like it too.

at first u take twice DEATH'S CARESS so u could have some good defense and in case they run or ambush u can make areal damage, and u take 1 GAZE so u could stun.
4 lvl u take another GAZE so u would have strong stun.
from now on only take ENRAGE and CANNIBALISM so u can have attack damage.
(all the way u buy items that give u HP so u don't get ATK.DMG at all that's y u get ENRAGE lvl 5).
after that u need to take GAZE till u get to lvl 5 and end the DEATH'S CARESS.
(u can change the order of them as u need according to the battle)

Well that's the tricky part. all the items are cording to the skills you take in the game.
at first Sheen, so u can stun and then give a strong blow. also use DEATH'S CARESS and then hit so u have 2 mighty blows.
then buy the PHAGE so u could DEC their movement speed and BOOTS OF SWIFTNESS so u could run faster then most champ's.
now u need to complete the FROZEN MALLET witch would give u 100% chance to DEC move speed and gives u more HP.
now u need ATK speed but also u want to cast stuns faster so u buy STINGER.
right now u are: fast, strong, DEC move speed of enemies and u give mighty *** blows after stuns.
all u need now is more ATK DMG and ATK speed.
so, u buy AT MA'S IMPALER that gives u DMG based on your HP and then WIT'S END that will give u ATK speed and more DMG every hit.

Well I'm not at a LVL that really know how to make this effective so I wouldn't trust mine here allot.