This post is for those newer players who have hundreds of options in front of them and gems to burn, not knowing where to start.
First and foremost, until you have built the maximum number of farms, upgraded them all up, and build and upgraded the various elemental temples, which legendary monster you obtain isn't going to matter too much. You're just not going to be that competitive yet (what's the point of a great legendary monster if you can't even level it to 100?). It'll come with time, just focus on that stuff for now.
So now let's say you're ready to start focusing on getting some great legendaries. What should you do?
So, Should I Get This Monster?
So, should you get that monster you've had your eye on? Well, if you're a collector and don't have it, then yeah, get it! But if you want to enhance your competitive standing (no matter what kind of player you are) you now need to understand whether that monster will be useful to you in combat.
With 3 monsters on your team, you generally want a controller/denier, an attacker, and a supporter. I may write up a blog post about that specific topic later, but briefly, you want your control monster to go first, stopping the enemy team from being able to attack, you want your attacker to destroy the health of the enemy, and you want your supporter to help you in the goal of winning.
So, when you look at a monster you're thinking about getting, you need to first understand the role that monster plays. Is he an attacker, denier, or supporter? If he's a denier, how does he compare to other options? How does he compare to monsters of the same role that are in the books that monster is in? You don't want to waste resources on the worst denier in a book, right?
Keep in mind, more recently released monsters are typically better than older monsters. The absolute oldest legendary monsters are almost entirely (but not always!) useless in the current game. Additionally, I do not recommend buying any monster from a chest, especially a chest that randomly gives you a monster from some grouping (except for maybe those Nemesis or Elite chests). I also do not recommend buying anything from any level up offer. Whatever you pay is not worth what you get.
One more thing: a lot folks like asking "which of these monsters is better?" The problem I have with this question is that different monsters fulfill different roles. They do different things, so comparing them is apples and oranges. Some monsters are objectively better than others, sure, but most questions we see on here aren't in that category. The better question is: what role does this monster fill? How does it compare to the monsters I already have that fulfill that kind of role (both overall and within the specific books)? And finally, how does it compare to other monsters I might be able to obtain?