The top two are really the only choices in my brain, but the rest are included just in case I am missing something.
Where to use mutant T4CC 26 votes
4* Domino (unawakened, but I have a mutant Gem and she'll be in Arena soon)
5* X23 (rank 2,unawakened)
Hold and Rank 4 Sabertooth (rank 3, unawakened) I'll be lacking T2A for this for months.
5* Nightcrawler (rank 2, unawakened)
5* Storm (rank 2, Sig 36)
5 Star Bishop (rank 2, unawakened)
5 Star Rogue (rank 2, unawakened)
Don't bother with any of the above and hold for later.