Destroyer39 said:

Oathbound, Rearguard, and Souldrinker are all decent arena champs, but I personally would choose Oathbound. Rearguard would propbably be next, and Souldrinker last. That’s my opinion at least.

I think I'd say Rearguard is the higher priority, depends a bit on where you want to use the champion.

But as for Oathbound, first of all his damage scales off defense, not attack. That's always a big plus on a champion, because it means that you can entirely ignore the attack stat and focus hard on defense, which will strengthen him both defensively and offensively at the same time. Secondly, he has an AOE decrease attack on a 3 turn cooldown which also reduces turn meter - quite powerful, will allow your team to survive longer in dungeons and potentially arena. And then the 4 hit A3 with block cooldown skills, also very useful. Make sure you give him high enough accuracy for all those debuffs. 

Rearguard is similar in several regards, though her AOE decrease attack has a much lower chance to apply. She doesn't have block cooldowns, but she has ally protection and a decrease defense on her A1 - making her a better choice than Oathbound for Clan Boss at least. Maybe also for dungeons, depending on what the rest of your team already offers. 

Thanks L9753! That was VERY helpful!!!