The following is my personal opinion on the best type of monsters.
1. Ghost
2. Pixie
3. Henger
4. Gali
5. Hare
6. Plant
7. Nya
8. Dragon
9. Magic
10. Dino
11. Tiger
12. Monol
13. Disk*
14. Golem
15. Doodle
16. Ape
17. Jell
18. Worm
19. Suezo
20. Naga
* I am predicting this monster's rank (I have never bred this monster before)
Ghost
The reason I think ghosts are the best is because I like smart monsters more than strong monsters, and I think the ghost is the smartest. The most easiest way to raise a powerful ghost is to do the mailman job until his Int gets to about 750-800. Then do the Circus job until you max in Spd. Then do the mailman job again until his Int and Ski are maxed. He won't need Pow because most of his attacks are Int based and his high Spd will make up for his low Def.
Pixie
Like I said above, I like to raise smart monsters. Pixies are second to ghosts in brains. You can raise a pixie the same way you raise a ghost (like I said above). If you ask me which pixie is the best, then I'll have to say the Angel (pixie/gali) is the best.
Henger
Hengers are probably the coolest monster there is in Monster Rancher. They can also be very powerful. The way I like to raise them is first, do the mailman job until his Ski gets to about 550-650. Then do the circus job until his Spd is about 600-700. Train Int, Ski and Spd until they're mastered. Then train in the following order: Pow, Def, Lif. If you combined 2 very good monsters to get a henger and he started out with Final Sword, you can train him like a pixie and a ghost (above) because he could just use his Final Sword to KO his opponent (since he has great Int and Ski). The best henger is probably the reformer (henger/monol) and proto (henger/gali).
Gali
Another smart monster. Galis also train Int, Ski and Spd good but they are good at Pow too. Again, the same method of raising a ghost works for this monster. They're also my favorite sub-type. The best gali is probably Pixel (gali/pixie).
Hare
Hares raise Pow and Int very good. But since I like smart monsters more, I like to train their Int more. Just use Deep Shriek at the beginning of a fight to KO your monster. I think Prince (hare/gali) and Groucho (hare/worm) are the best hares.
Plant
Plants are also good with Pow and Int. Even though I'm not real good at training plants, I like to train them to have an average of Pow and Int (Int being higher). They can also be trained to have high Int and just use his Machine Gun. I'm not sure on the best Plant type, though.
Nya
Although they only have one Int attack (beam), I still like to train their Int while neglecting Pow. As you have might of guessed, I just use his beam attack to win. Lovey (nya/tiger) is probably the best.
Dragons
Dragons can be either trained by Int, or by average (Pow being highest). If you train Int, you could always use Body Slam to win a battle. But trainig Pow lets you have more attacks. The Gariel (dragon/gali) is the best.
Magic
Magics are quite similar to dragons. They have a majority of Pow attacks while they can still do quite well by just trainig their Int. I'll have to say the Gangster (magic/henger) and Kaduka (magic/naga) to be the best Magics.
Dino
Dinos can be raised intelligently and powerfully. Just use Flame Tackle for Pow and Fire Bomb for Int. The best Dinos would have to be Lidee (dino/tiger) and Goldy (dino/gali).
Tiger
Although Pow and Int works for tigers, Int is probably better since it's their long-range attacks that are Int. Velvet (tiger/monol) is the best Tiger.
Monol
Even though monols are made to train defencively, I still train their Int and Spd the most. Triple Beam does the trick (as well as Formation, being a monol's strongest attack). Scribble (monol/???) is probably the best Monol.
Disk
??? (Radial (disk/???) is probably the best Disk cuz of their Spd)
Golem
Golem's can be either trained Int or Pow. While having mostly Pow attacks, you could train Int and just use Large Quake. Amenho (golem/gali) and Dean (golem/pixie) are the best.
Doodle
If you ask me, all Doodles are is that they look kinda funny. Pretty everything else about them suck. Mainly because of their attacks. Most of them are D Hit% and Damage. It is still possible to have a powerful Doodle, but then their stats will need to be really good. Purebred and Sketch are probably the best.
Ape
Apes are probably the most difficult monsters to breed. So far, I have not raised a successful ape before. I don't think there is a "best" Ape, but I think it's Great Ape (ape/gali).
Jell
Jell are like monols, but quite worst. I just stay away from raising them and anyone with a jell sub-type. A pure Jell is probably the best.
Worm
Worms are probably best to be raised powerfully since they're not as good with raising Int. The best Worm would have to be the purebred Worm.
Suezo
I don't know why, but I HATE suezos. I just think that they SUCK! They're hard to breed and by the time you have *actually* made them into a good monster, they need to be retired! Forget it!! There is no "good" Suezo!! They SUCK!!!
Naga
Nagas are probably the dumbest monsters (and that should explain why I think they're the worst). Pure Nagas are probably the best.