When players use a Titan, there’s often 1, sometimes 2 abilities that are commonly used and the others are not used as much. So you only need to really remember that ability and the Titan’s ongoing effect/Battlecry.
The Primus (Death Knight): Destroy an enemy minion. This minion and your hero gain its health. Then also discovers a card with that rune (in this case, Blood).
Argus (Demon Hunter): Summon four 2/2s, two to the left (which have Rush), two to the right (which have lifesteal) OR Reduce the cost of all minions in your hand by (2).
Eonar (Druid): Easy one to remember, either refreshes all your mana, heals you back to full, or draws until you have a full hand.
Aggramar (Hunter): Players often go for giving their weapon +2 attack and Immune while attacking, making the weapon they get a 5/3 off the bat.
Norgannon (Mage): Norgannon’s damage ability doubles from 5 → 10 → 20 after using the other two abilities.
Amitus (Paladin): 2 is the magic number here, draw 2 set to 2/2, buff minions +2/+2, set enemy minions to 2/2. Ongoing effect is your minions can’t take more than 2 damage at a time.
Aman’thul (Priest): Choose two enemy minions, remove them from the game. Discover a Legendary minion (from any class). The other 2 abilties summon “stuff”.
V-07-TR-0N (Rogue): Simple way to remember is they buff a friendly minion and give an effect of either: deal 4 damage to a random enemy, draw a card, or give “can’t be targeted by spells or hero powers”.
Golganneth (Shaman): Deal 3 to enemies + heal 6 to friendly characters. Your first spell each turn costs (3) less.
Sargeras (Warlock): Opens a Twisting Nether portal that summons two 3/2 imps each turn. Abilities: basically poof all minions into the Twisting Nether, or give future Demons summoned from the Twisting Nether +2 Health and Taunt.
Khazgoroth: He starts off as a 4/4 and can grow to a 9/4 Immune that attacks an enemy minion or a 4/9 Immune that attacks an enemy minion. If he gains attack, you gain attack, if he gains health, you gain armor. The third ability I hardly see used.