Yeah, most of then are 6* worthy, but also all of then need books to get most of them - some of them have to be fully booked, some of them have to have their important skill CD lowered.
Kymar is your best Arena investment - Platinium worthy, he need atleast his A2 and A3 lowered. Top lego.
Steelskull is very good CB champ, he have to be fully booked and properly mastered to start to shine tho. Definitely not the best choice as you second 6*, but keep an eye on him.
Apo is great, but he can work well enough as 5*, so not your optimal choice for 6* right now. Just give him good speed and survivability, lower his CD's and use him like this for now. One of the best rares.
Stag knight - can't realy say anything bad for that guy, you will 6* him at some point, as he will bring value in almost every content. Almost close to first place here :)
But as for second 6*, MM is probably your best choice. This means your first 6* is your starter, I guess. For some time you will have one damage and one support until you get your next 6*. MM will carry your weak 5* champions through content, otherwise you will have very slow progress until you get more 6*s.
Not the best choice for CB, but your key to progress on all other places. Build him properly and use him to cary your good, but still weak champs. MM, your starter, Apo and Stag Knight will be your defaul comp fot some time i think.