One of the only games I've picked up and decided to get 100% achievements before I even play it. I do not regret it one bit.

I will say that this game isn't perfect. The difficulty spikes are rough, the level design can get just a little tedious (though that's more the fault of search action games as a whole), and I could not confidently tell you anything about the story or lore, but the rest of the game more than makes up for it.
The combat is amazingly in-depth, and every monster can be customised so many different ways, to match your play style or to work in a unique team composition.
Collecting every monster is also engaging, in that you don't simply catch them. I won't list them, but there's also alternative methods you must go through to get some of them. You'll find that getting every monster collected will be more engaging and challenging than you expect.
There are two types of battles: Tamer battles, and Wild battles. You won't find Monster Tamers waiting to meet eyes in the field, but wild monsters are scattered around, ready for you to battle if you contact them, making it easy to pick your battles. The tamer battles are all either story important or vehicles for unlocks, so they're still worth doing, and are worth the extra challenge.
Finally, the pixel art is wonderful and charming. I am VERY picky about pixel art usually, and this game ticks all of the right boxes. There's nothing that's especially spectacular, but the game ends up looking beautiful nevertheless. All of the monsters are extremely charming and its easy to find one you feel especially attached to. (UPDATE EDIT: After a recent free update with a ton of content, a load of the lower quality sprites got massive overhauls, making an already pretty game even prettier. Huge props to moi rai games for this!)

This game is a wonderful blend of monster taming and search action games, two genres I absolutely adore. If there was a Monster Sanctuary 2, I would buy it in a heartbeat.

UPDATE: After more achievements and extra content were added, I went back and 100%'d the game again. I still totally stand by everything in this review.