Kael, Tayreal, Apo and Skullcrusher are your top. Most of all Kael first to 60, then tayreal or apo

id do kael, apo, tayrel, then skullcrusher 

kael will be your campaign farmer so get him 60 first to speed run through campaign levels and get fully mastered eg fully scrolled to tier 6 

apothecary is a great healer and can easily step into spot 2 and carry kael with speed boost and healing if you want to hold off doing tayrel.

skullcrusher provides counter attack and having him will provide extra turns in clan boss and help drive your damage up - if you can pull frozen banshee she will kick kael out off your team easily frozen banshee when ever you pull her is a CB beast and dont leave her in vault sitting there.

tayrel will help you in so many areas including cb with decrease defence and decrease attack 

tayrell can also farm Campaign ( not the fastest ) but pretty reliable.  my suggestions in order : 

I would first go Tayrell, Apothecary for your general progression. 

 Skullcrusher for clan boss and fireknight. And mausoleum Mage for dungeons, arena and also Clan boss. 

Jareg is kind of redundant in CB with tayrell already there but can still help in dugeons liek dragon, or even in cb as additional ally protection in conjunction with Skullcrusher , will serve also as backup attack down. 

Psylar is great in dungeons and faction wars, and even in arena but I'd leave her for last as she can still work pretty well at level 50 with good enough gear. 

Best of luck.

I'd say Kael. Your 1st 6* should be a champ able to solo the campaign so you can use him to lvl up champions for future 6*s . The rest are solid champions but I feel like Kael will do it faster

The difference between Kael and tayrell when farming brutal 12-3  is about 10-15 seconds per run at most. So I'd still go with tayrell. However Senda's suggestion is also sound, especially since Rares require much more affordable rare books to max out.  ( as well as less tedious potion famring ) 

My vote would goto Kael if you havent all ready choosen :)