Your first 6* always should be your starter champ. At least 99% of the times anyways. So go for Kael. The reason is fairly simple: it's much easier to build a rare champion in terms of books than an epic champ, or especially a lego. 

So first you 6* Kael, who will help you in about all parts of the game: CB, dungeons just to say the most important ones. 

You may think that epic champs are inherently better then rares, but that's not always the case. It's especially important in the beginning to go for champs that are easier to build, and can be used everywhere. Kael is such a champion. The only argument in your first 6* now would be wether you go for Kael or Elhain, but Kael will help you in more areas in the beginning, since he brings poisons.

The next 6* after Kael is up for debate, but I would suggest to build up Doompriest. If you build him fast, you can stack up his passive heals pretty well. Also he doesn't need to be booked to be usable even early on.

The ones that you really want to 6* are: Kael, Doompriest, Demytha, Genbo, (Ronda, Elhain). The rest are not really worth it, or they can be used at lvl 50. Like Hoskarul is a pretty decent CC champ, so you want to use him to stun waves, or in the arena, but he can do that on lvl 50. 

So yeah, these are my thoughts, you can take them, or just think about them

Thanks Wandor