I have been using a new Untamed card that has the ability- Opportunity and found it doing exceedingly well. So well in fact it feels like the ability is a better choice than others by a long shot.
The $ cost to obtain a card with this ability is low considering most base monsters with this skill are commons.
In my opinion Opportunity trumps abilities such as Sneak and Reach. A direct hit to the weakest opponent seems a lot more useful than hitting the last opponent or being able to hit when standing at second position. Of course the stats: power, speed, toughness, and health comes into play with these abilities. However when comparing ability to ability, Opportunity simply is more versatile as it can likely hit where ever the opponent position's his/her lowest health monster. Also the monster with Opportunity can be in at any position in the formation and still be able to attack opposing monsters.
Sneak & Reach
Dragon Jumper is definitely a card to watch out for. With a speed of (5) it will likely be able to attack the opponent before they can react. Then it should be safe to assume when having any one of these monsters leveled up it will become a dangerous adversary. Look at the stats below for only Serpentine Spy has max speed reached up to (6). The other monsters with Opportunity have either increases in melee, heart or additional abilities.
Serpentine Spy- Fire
Great Monsters Combo with Serpentine Spy?
One would be looking for fire cards with health+ and toughness+. There is Lord of Fire and Magnor respectively.