lAll of these monsters can be recruited during the course of the game. To recruit monsters, you must have a Beastmaster and the wagon must be available. You must have at least one empty slot to recruit the monster.
Drago is a slow leveler but is still a powerhouse. You can only get Drago rather late in the game so she'll probably be pretty far behind the rest of your characters in both jobs and levels. If you want to spend the time getting Drago up to speed, it'll be worth it.
Personally, I don't like Rocky. He has exceedingly low MP despite his powerful techniques. However, he is greatly handicapped since he can't reach higher than level 30. Rocky quickly outlives his usefulness. Rocky gains levels very slowly to put it lightly.
Boss Trolls are very strong and have tons of HP. However they are very muchly lacking in the MP department. Don't even bother trying to make these guys into magic users. Stick with the Soldier/Fighter path, and you'll do fine.
Dark Horns aren't the most useful of monsters, but they aren't necessarily bad ones either. They learn very few skills but do have excellent HP, Strength, and Defense. Overall, this monster will become another fighter-type.
Furrats, like slimes, are good at the accessory classes since their stats stay relatively low. Their style, however, is higher than any other monster. For the monster competition in the Style Arena, Furrats make relatively good candidates- only Slime Knights can do better due to the Platinum equipment bonus.
Healers can be very useful- they make perfect priests and are the only person/creature in the game to learn HealUsAll which costs a steep 36 Mp. Since you can get one of these very early, I recommend getting one and placing it in the wagon for healing on long trips.
Killer Machine 2: Robin
Killer Machines have very high strength, speed, and defense. Robin won't get much in the way of MP so magic using jobs aren't the best option for him. His naturally learned technique selection is rather slim too.
Kings has both high HP and MP with some very nice stats to boot. Kings can specialize in pretty much anything that you want him to. He's good at everything, but not overly outstanding in anything.
Kadaboo is probably the best monster you could ask for. The only major drawback is that you can't get Kadaboo until you cross the Sea of Nothingness in Deathtamoor's Realm. Kadaboo makes an excellent fighter and an excellent mage. His strength starts out exceedingly high making him just that much better.
Lesser Demons aren't particularly spectacular. Their Wisdom and MP are very low so they should stick with the Fighter-based classes. They do learn some very helpful spells naturally, but, overall, they aren't anything amazing.
The Lipps starts out with relatively low stats, but it gains a fair amount of Strength and Wisdom and very nice amounts of HP and MP. Since it can only advance to level 50, Lipps don't prove very useful rather late in the game, though they can make fairly decent magic users early on.
Getting one of these guys takes enomorous amounts of time. Metal Babbles have insanely high defense and agility- it starts out with 500 of each. It doesn't have many hit points at all though, but it doesn't really need them either. It gains only a few skills since it's maximum level is only 16. The Metal Babble can make a very good contestant for use in the Slime Arena.
Jimmy is a fairly well rounded monster. He has a nice amount of HP and Wisdom which can make him both a good fighter and a good magician. I recommend building him towards a hybrid job such as a Paladin or a Fighter-Mage to maximize in both areas.
Smith starts with a nice chunk of HP. He can become rather strong, but I don't find him too useful since he can only reach level 50. If you really want to use him, build him towards the Battlemaster path.
Rookie is alot stronger than your normal slime. He also comes with a couple spells. However, by the time you can actually beat the H class, Rookie is rather outdated. He's still very good.
Slimes are very weak, but they do well in accessory classes like being a Thief. I don't know why, but I love these little guys.
Slime Knights are very well rounded monsters with good magic and power potential. If you need help winning the monster portion of the Best Dresser Contest and don't feel like getting a monster up to some ungodly level, Slime Knights can win it rather easily since they can equip the Platinum Equipment for a nice +50 bonus. Pierre also gains levels relatively easily.
Super Tensks are natural dancers. They learn most of the dances as they gain levels. All of their stats are relatively high making them good at anything you decide to make them specialize in. If you need another strong fighter, you can make them into one. If you need a wizard, they can do that too.
Melby is a Wind Mage and is very good with magic in general. He learns all the Infer-based spells naturally. His MP is quite high too. Melby works best as a Sage.
Mecky is a very good monster to get. His stats are all very nicely balanced, plus he gains levels very quickly. His highest stats are his Strength and Wisdom. Mecky will end up doing very well in any job you want to give him.