Who to R5 First?

I have just completed Act 5 for the first time last week. At the moment. I am focusing on trying to Explore Act 5.

I just aquired the 2nd T5 basic needed to R5 a 5*. Who should I rank up?

I've been thinking it will be Doom but I don't know how useful he will be moving forward.
Even if I get a good 6* for my first one (in about 2 months at my current rate) I expect my 5*s will still be my primary focus for quite a while.

I have Skill/Science/Cosmic AG available, if that could be relevant to your answer.

Comments

Anthinho

Doom will probably never lose his use if you know how to play him, even in act 6

Jetenyo

Update:
Thank you to all who answered. Clearly Doom was the way to go. I did end up following the advice given and surprisingly (with the help of R5 Doom) have pushed my way the whole way up to 6.1.5 in a very short span.

I also managed to get my 6* sooner than expected and it was Mr. F. I have had many regrets with ranking champs in the past. I am happy to say this will definitely not be one of those cases.