Borongo is quite powerful, but he doesn't start gaining any skills until much later on in the game. Overall, he's a very useful character.
Attackbots are very powerful which makes it for their complete lack of Skills and Spells. However, their max level is rather low, which causes some problems- especially since you can't recruit them until the end of the game.
The Big Eye is a monster that I don't think is overly useful. They have a low max level, not very good Skills, and aren't overly powerful. I don't think that it's entirely worth getting them, but they are rather easy to recruit nonethless.
Blizzard Hawks actually have quite nice statistics though their max level is low. They're rather useful for a while though with spells such as HealUs considering they do more damage than Healers.
Bomb Babies are only good when you get them- and only good briefly at that. Their stats are above your current monsters' averages most likely when you first get them, but they level rather slowly. They become worthless after almost no time.
Bombcrags take forever to level it seems. Their stats aren't worth the wait, plus, they overall just aren't very effective in battle. Sacrifice isn't worth what it entails either really.
Brownies are decent- they has no spells or skills so they rely on physical strength alone. They have a limited selection in weapons which doesn't help too much either. Use them if you wish- I didn't find them very helpful after Reinhart.
The Centaurus is definitely a good monster- having both Bikill and Barrier is exceedingly helpful since it negates the need for 2 different people to cast those spells later on. They also have HealAll making them even better. I highly recommend using themm.
Curers are very useful, getting many of the very important healing spells such as Revive and HealUsAll. I'd recommend getting one and placing it in the Wagon if you don't want to use it in your main party.
Dancing Jewels' stats go up quite a bit with each level due to their low maximum level. They're useful for the Volcano near Salabona, but that's pretty much it.
Though the dance attacks are rather interesting, that's about all Dancing Needles have going for them. The stats aren't too high, and none of the skills are really *that* useful.
Dragon Pups are decent- nothing to spectacular though. Their breath attacks will come in handy for a while though.
Drakees' are poor statistic wise. They don't get anything to fantastic spell wise either other than BeDragon. Their main advantage is that they don't have a max level.
Eigerhorns are definitely worth getting- they are very strong, have tons of HP, and just have nice stats all around. Their skills are very helpful- including War Cry. I highly recommend using an Eigerhorn in your party.
Eliminators are purely physical fighters. They learn one spell- Sap. Massacre, however, is much more of a danger than an asset. I don't recommend using Eliminators- there are much more useful monsters out there that can do more physical damage.
Farewell Crags are the hardest monsters to catch- harder than even Metal Babbles. However, they just aren't very useful. Farewell is nice as a last ditch tactic- but that's it. Their stats aren't too spectacular either. You're better off with an Eigerhorn really.
Gigantes are powerful physically, but that's their only advantage. Their low max level reduces their use quiet a bit, as does their complete lack of skills. You get them very late in the game too when there are many more monsters that are better which you most likely have.
Golems are quite useful- they're strong enough to make a difference in the party, and they don't have a low max level. Plus their skills fit in well to their class.
Great Dragons are definitely worth recruiting. They have all the breath attacks, plus their stats are amazingly good. Their attack power is just beautiful. If you can afford to spend time leveling them, they'll prove to be worth it.
Healers are very useful monsters. They gain all the important healing spells so they'll prove to be useful. Since you can get them early, they'll be a very high level by the end of the game. I definitely recommend using one.
HellBattlers are nice due to their great skills. Their stats are also very good, but the main problem comes from the fact that you can only recruit them in the Bonus Dungeon. By this point, you generally already have a definite party formed out.
Horks just aren't that useful. Their skills aren't powerful at all, and they can't use much in the way of equipment. I don't recommend them.
King Healers aren't as good as Curers or Healers- their max level is lower, and they have no way to heal only one person. Their skills are still rather useful though.
King Slimes are rather useful monsters with some interesting skills. I'd recommend getting one for some of their support spells such as Freezing Rays and Barrier.
Kukkles learn some decent support spells, and are useful when you can first recruit them, but as time goes on, their usefulness diminishes.
Leaonar's aren't bad, but they're not particularly outstanding either. There are better monsters you could probably use.
Lionexes are good monsters, but since you get them so late in the game, there are better monsters you could probably put into your party. But if you want to invest your time in developing a Lionex, you will have a powerful ally.
Mad Dragons are good, and they learn some useful offensive skills as they level up. Not a bad choice, but there are better monsters too.
Magicians are fairly good at, well you guessed it, magic. They have a large advantage due to the fact they don't have a level cap. Their spells aren't the most powerful, but they are still exceedingly useful.
Messalas are all around useful monsters excelling in both physical and magical attacks. Their strength and nice level cap give them ample opporunities to grow. I recommend using one when you can get them on the way to Granvania.
Metal Babbles have absurdly good Defense and Agility, but they lack HP. They have some very impressive offensive spells and are worth catching if you wish to dedicate the time to catching one.
Minidemons have some strong spells later on, but they are not useful as a whole. Their low statsdefinitely don't make them worth time in investing unless you want Explodet- even then it cannot be used very often.
A Nereus makes a very strong magic user and is much better than Magicians when it comes to a purely magical monsters. The amount you use magic determines how useful a Nereus will be.
Orc Kings have tons of HP and a large amount of strength too. Though they have some nice skills and spells, they aren't overly useful at the point in the game which they become available.
Puppet Men have some nice skills though they aren't very powerful all around. Their low max level interferes with them becoming overly powerful.
Slimes are poor fighters and not the best suited for magic either. Their main use is only for support magic.
Slime Knights are excellent monsters. They can equip the Metal King Swords which you can get rather early making them absurdly good fighters for the beginning of the game. Couple that with their nice healing spells, and you're set for quite some time. I usually have one in my party the entire game.
Soldier Bulls are exceedingly strong and have a few useful skills such as Bikill and Charge Up. They can also equip a rather wide range of powerful weapons and armor.
Springers are fairly good- they have nice attack and defense abilities, but their skill selection is very poor.
Wyverns are excellent monsters having both good offensive and healing skills along with some helpful field skills. With a nice level cap, Wyverns can definitely make a good addition to the team.
Yetis are decent monsters- they have some nice skills and also a good balance of strength. It'd be nicer if better equipment could be put on yetis, but you most likely won't use them for very long anyways.