Alright, I've thought it through for very long and have come up with a tier list of summoner spells for Dominion. There are neglected spells in Summoner's Rift that are much much better in Dominion than ever thought of before.
The main goal of Dominion is capture and hold. Capturing points continuously damages the enemy nexus depending on how many are captured.
It is also wise to push minion waves to enemy capture points and to traverse the map as fast as possible to capture vulnerable points and to avoid both enemy captures and confrontations.
So here is the tier list of spells best suited for Dominion and my reasoning behind them. They are rated in the following order.
OU: Overused. Spells that I would recommend you take at least 1 of per champion
BL: Borderline. Good picks for Dominion's rapid paced gameplay
UU: Underused. Spells I would not recommend for Dominion
NU: Never use. You best not choose one of these for your own good and the good of the team.
Revive: Best spell for this map. In Summoners Rift you would get flamed for feeding the enemy extra gold, on Crystal Scar champion kills are far less important as staying on the map capturing points. With the mastery you will be able to survive longer and push lanes harder making it very good for this game style.
Garrison: At first I never used this spell because I thought it was terrible. One reality check later I found it to be a lifesaver against push teams. The duration is much longer than I anticipated and it can be used to recover a point very fast if an enemy attempted to capture it or push a capture point across the map.
Ghost: Ah Ghost, the classic getaway/initiate spell now making Flash obsolete in this game mode. There are tons of opportunities in which you can use this spell and it will allow you to chain capture like a boss after an ace. Very powerful spell.
Exhaust: This spell is great for champion kills and even better when trying to make a getaway through a speed shrine. It shines in any game mode and in Crystal Scar it has practically the same impact as it does on Summoner's Rift.
Heal: Very good spell for this game mode where support plays a big role and staying healthy plays a big factor in capturing points while in enemy territory. Healing during a push or after a team fight will allow you to capture harder which is why I recommend it.
Promote: Anyone remember the old Promote? Meet its revamped, beefed up cousin. Put this on a siege minion and watch as he crumbles crowds of creep while giving you gold. This spell is a must on push champions like Heimerdinger and Alistar as it allows them to gangbang a lane very quickly with minions.
Ignite: This spell is simply laughable on this map with health relics scattered around and passive buffs that give you a regenerating shield. Its good if you want to get last second kills and first blood but I'd rather leave the slot open for a better spell.
Flash: Three words breaks this spell: capture and hold. Escape with this spell is now meaningless since if you ran away you already lost the capture points to the enemy, putting you further towards defeat. Initiation with this spell is now useless as the long cooldown, lack of walls to jump over, and fast paced gameplay take a lot away from its charm. Its the best spell ever for Summoner's Rift and Twisted Treeline but not Crystal Scar.
Clarity: Mana problems should never be an issue in a game where you start at level 3 and can buy any powerful mana/mana regen item you want. Seriously, I highly doubt it will be of use on anyone thanks to chalice being available at start.
Clairvoyance: Vision of the map is so widespread that you can often times tell where the enemy will be heading before they show up. Wards are disabled on this map so that should be a good enough indicator that you don't need map awareness to play Crystal Scar. If you pick this spell I will queue dodge and write a 600 word essay on how terrible your choice was (I won't really).
Try these recommendations out yourself and leave a comment below. I hope to play more Dominion games in the future to further improve my analysis. I will also be writing a guide on Dominion Items later on so stay tuned and happy hunting summoners.