I think, there are arguments for 3 different options.

1. Warmaiden. Decrease def on all enemies helps nearly everywhere in the game. And you can farm copies from Warmaiden in campaigne to increase her skills (copies of a champ can be used instead of skillbooks), making her a "cheap" way to increase your account.

Downside: There are a lot of other champs with decrease def in the game, who bring more on the table in addition. For example Stag Knight: decrease def on all enemies AND decrease attack on them. Or Uugo: Decrease def AND block buffs on all enemies, and heal and revive and leech. Or Dhukk, Armina, Tayrel, Deacon Armstrong... In the end, you will replace Warmaiden with one of these and she will be a Vault-inhabitant.

2. Bellower. All of his skills attack all enemies, making him a good candidate for a Stun-set. In addition to that, he has a ton of usefull debuffs at his kit. Block active skills on the A1, decrease speed on the A2 (usefull in a lot of bossfights in dungeons), decrease attack and decrease def on the A3. I have a lvl 60 Bellower in a Stun-set on my own account. His A1 helps a lot to beat some nasty enemies in dungeon and DT waves. 

Downside: on the A3 only the small version of both debuffs (he would be to good for a rare otherwise).

3. Wait a few more days, you get High Khatun as log-in reward at day 30 and could make her lvl 60. At the beginning of their Raid career most players underestimate how important speed is in this game. Khatun has a speed aura, a speed buff and increases the turnmeter of your team.

Downside of making her lvl 60: she can do her speed stuff the same way with lvl 40 or 50. Extra levels would only increase her survivability. Maybe you don't need that right now, unless you fight dungeons stage 20.

Skadi, thank you for your exhaustive explanations.

I'm personally also considering between Warmaiden and Bellower. I use Warmaiden nearly overall (CB, sometimes Arena, DT). From other side Bellower seems to be much more useful for DT (but useless in CB).

Speed aura of Khatun will work without levelling her up. And concerning other level-dependent abilities - she has not impressed me in other teams so far.

I agree with Skadi about the three options, but would think both Warmaiden and HK can do their job fine at 50, leaving Bellower as the best candidate. He's perhaps not the ideal third 6*, ideally you'd like some solid support champion, but you don't have those. I'd say take Warmaiden to 50 first, then Bellower to 50, and if you still don't have better candidates for 60 by then, Bellower to 60. 

1. WM

2. HK

3. Bellower

IMO - none of them. When you get HK, get her to 60 - she'll last you until Arbiter. But I wouldn't bother with WM or Bellower. Just level food until you get better epic/legos.

Honestly, i see only 1 advantage of HK comparing to my fully-booked Diabolist - Aura 19% Speed. But it is mainly useful in Arena-offense and i don't need level HK up to use her Aura. In other aspects fully-booked Diabolist seems to be slightly better.

Or maybe i miss something?

The aura is 100% worth it. That aura is the difference between a win or a loss in arena fights. Also, Khatun's speed boost is coupled with an increase speed buff, whereas Diabolist has that on two separate moves. While it's true that Diabolist also has a TM decrease on her TM boost, that requires accuracy, which makes it harder to gear her. Lastly, for dungeons, Khatun's speed boost is on a 3-turn CD, as opposed to a 4-turn CD on Diabolist.