As you fight, you gather a certain amount of something called "Finesse" - it's what you use to trigger your Ultimate, as you probably know by now. This is a short list of all those abilities and their benefits for the group.
Dragonknight
Unlike everyone else, Dragonknights are very slow to gain ultimate on enemies, unless using the Fighters Guild passive and fighting Daedra and undead. The quickest way to gain it seems to be the Earthen Heart Skill line, but the experts I talked to weren't too clear about how.
Their ultimates are very useful, though.
Dragonknight Standard - Morphed: Standard of Might
Provides a huge damage boost to the whole group, while at the same time reducing the damage income. Synergy Shackle damages enemies in the area.
Dragon Leap:
Quickly takes the Dragonknight from one point to another. Not too useful for groups, unless you play throw-the-knight.
Magma Armor - Morphed: Magma Shell
For the tanky knight, or the one who has to constantly resurrect his wife. You become almost invulnerable for a short time, and allies who use Protective Shell get a powerful damage shield.
Nightblade
Death Stroke
The Nightblade does nasty things to enemies low on health, and even gains increased Finesse while on a killing spree. Almost useless on bosses, since they usually are not subject of killing sprees. No synergy.
Soul Tether
Leeches life essence from the nearest target, stuns enemies in proximity to the Nightblade. Synergy: Soul Leech - lets you leech on the enemy as well. A clear #2 for the DPS output.
Consuming Darkness - Veil
Creates an area of darkness for 12 seconds. Enemies in this area are snared 70%. Nearby allies take -30% less damage. Synergy Slip Away makes you invisible, so you might as well not use it.
The tooltip says it all - a powerful damage reduction on command. Very, very useful.
Sorcerer
Summon Storm Atronach
Everyone knows this one, because these things are almost impossible to overlook. Not too useful in trials, but a charm when you're about to get devoured by a gazillion tiny adds. Synergy: Charged Lightning, causes the Atronach to deal 150% additional damage.
Negate Magicka
The beloved "Negate" that people always yell for. Dispels enemy effects in the area and silences hostile casters - just what you want when you have some aggressive s'wit running around tearing yor group apart. No synergy, but it doesn't really need one.
Overload
Turns the Sorcerer into Iron Man and lets him shoot lightning from his fingertips. It's a bit like a Redoran Festive Armour, fancy, but entirely useless.
Templar
Radial Sweep
Stabbity action with an Aedric spear. Although it looks really competent, that's about all it does.
Nova - Morph: Solar Prison
Enemies deal 30% less damage for 8 seconds, and take 9 Magic Damage every 1 second. An ally may activate the Supernova synergy, dealing damage and stunning all enemies in the area.
My most favourite Ultimate of all times. Not only does it look impressive, no, enemies also deal less damage while it's active! That even counts for end-of-the-world killing blasts... It's a lifesaver if it's up at the right moment.
Rite of Passage
The templar becomes the centre of a healing aura which may or may not get the group through a heavy AOE impact. Although it doesn't influence the enemy's damage output it may bring the group through a nasty situation that they would otherwise not survive.
