Take my advice with a grain of salt , im still learning (closing in 6 months)

But you cant go wrong with booking deacon . 

Wait for more replies to pick between answer since mine may be sub optimal since i do not know a lot of champions you named :)

Deacon is a solid champ.  Without more context (anything in particular you're wanting to work on?) I'd say he'll go far for you.

The short answer is none of them. Don't book anyone until you:

1) Have them at 60

2) Have a team you've identified that needs them

3) Have determined that they need booking in order to be used on that team

4) Have concluded that you don't have anyone else that could fill that role eventually, but are waiting to level

i

Keep those circled ones, and you can feed the rest. Not sure about Arndulf and Jurojin, never used them. 

As to which one to book next, I vote definitely Deacon. He can be run at 50 for a long time, but he does need books to be really effective.

Would it be possible to see your entire roster?

Kram's approach is usually sound but Deacon is just so incredibly strong - and in so much different content - that he's going to slot into your teams for a long time and really does need those books.

Couldn't get it all in one comment, but this is what I've got that's of any use to me so far. I use all of these for FW or dungeons. 

I would be windmill slamming my books on Deacon.

I'm on Team Deacon.

Time to rank, level, rank, level, ascend, mastery and book Deacon.

I wouldn't book anyone...

12.3 brutal.... 6star deacon then book

So...

Wait two days.... Then book Deacon?

In case I get a better champion? Or...?

ok. there is an argument to be made on the proceedure to train champions.  Masteries, books, levels, etc.  there is definitely a 'most efficient' way to do it.  

But given the question you asked, and given your stable of epic champs,....

DEACON,  yes, get him to 60, yes, get him masteries, yes, book him.

The other question is would one of your legendaries take precedence?  But you asked about epic books, epic books will not help you develop your leggos.

This.

You want to level to 60, ascend, mastery and book Deacon. Order is slightly less important here than that you go ahead and do all of this and have him done. :)

You'll book Big Deac.....whether it's today or tomorrow depends on what content of the game is important to you, but based on your roster right now...Deacon. I wouldn't even wait til 60, you can start using him as your turn meter booster AND a debuffer now. Speed, Accuracy, Survivability. I'm struggling to think of an epic I'd prioritize booking over him in early game. He'll serve you well into late game, but by then, you'll be eyeballing Hydra, Iron Twins, and Doom Tower, so Husk, Seer, Shamael, Geomancer will make more sense to you, but you cannot go wrong with Deacon. 

I'm not sure about Deacon's speed prior to 6* ascension, but I doubt it matters as much in his Arena tier. I believe his A3 grants an extra turn when ascended, so I'd say getting Big Deac to 3* ASAP was the right choice, and I do think booking his A2 is the next priority for 100% defense decrease....after gearing him right, of course. And he can start rocking him as his turn meter booster and debuffer at 40. Unless he's duking it out in high gold arena, both a turn meter booster and a debuffer in a speed team depends more on gear than anything else. 

The main challenge would be gearing him with speed AND accuracy. Speed boots, Accuracy chest, SPD, ACC, or Percepton sets, and he's ready. I've been beaten by teams with lvl40 Seeker/Khatun and Warmaiden because I looked at team power and thought I'd run through them. My Seeker was lvl60, their Seeker was faster....that's all it took. 

I would go with deacon

Deacon for sure. rest is... the rest

I use HellHades for masteries. I have nothing to replace Deacon with atm. I'm assuming Speed Lead build until later? Going for Arena Challenges and I need to break into gold. I'm sitting in Silver 4 with Deliana, UDK, Kael, and Fenax. Who should I replace?

I'm thinking keep Deliana and UDK for sure, but unsure of the other two. Both do great dmg, but lack survivability. Kael is a little faster atm, Fenax has atck% boots that I've been trying to replace for a while. On the other hand, Fenax dmg is better on his A1 plus the block revive. 

Any suggestions?

what kind of team are you trying to build here? if it's a standard speed nuke setup, then the correct answer is all of them, but keep either fenax or kael around until you pull a decent dps.