Heartbreaker Series[]
This series of monsters' active skills are to change Heart Orbs into their own element's orbs, exchanging recovery for damage output! All of these monsters make excellent subs for various teams due to their buff evolved stats, dual colors, and ease to skill up from Poring Tower.
Siegfried, Cu Chulainn, and Valkyrie's new Ultimate Evolutions give them a tremendous boost in value, but they also require a huge amount of EXP and materials to evolve into those forms. A new player is not advised to start with their base evolutions, though the evolved forms are slightly worth considering.
All in this series can be obtained outside of the Rare Egg Machine. 
Red Giant Gigas and 
Duke Vampire Lord can be farmed through Pal Egg Machine or early dungeon drops. 
King Siegfried, Blue Champion and 
More Cu Chulainn, the Hero can be gained through The Heroes' Hideout. 
Princess Valkyrie can be acquired from The Goddess Descended!. If you pull them, you should probably reroll.
Note that in Version 6.2, alternate-colored Valkyries 
have been added to the Rare Egg Machine, but they are functionally identical to and fit with fewer teams early on, so they are even less recommended as starters.
All monsters (except the alternate-colored Valkyries) in the Heartbreaker series have been removed from the REM as of March 19, 2015. The info here is retained for reference only.
team because of -500RCV. Can be used in many teams such as a 
team as a stat stick and orb changing. Can be farmed early on in Tower of Giants, so no need to start with him.
and 
, can cause an obscene 209.25x attack! It's also easier to form a mono-color Balanced Team. Dual Wood/Dark colors are also very valuable in an Egyptian team. A new Ultimate Evolution gives 3.5x ATK to Balanced types above 80% HP but requires running a very advanced dungeon.
and 
can cause a massive 209.25x attack. However, Healers' low HP and ATK are still a problem.
team. Once ultimate-evolved, can compete with 
and 
as a leader, but it takes a while to get to that point. Alternate evo 
has a stronger Leader Skill but is far out of reach for a new player. He can be farmed from the Pal Machine or Castle of Satan, so don't keep as a starter roll.