Very tough one, but I would choose Nick Fury and Cosmic Ghost Rider, since they are not available as 7*, and will probably take YEARS 'til they do. Every single time i upgraded a 6* past rank 3 and ascended, shortly after i received the 7* version. Happened with Gamora, Overseer, etc.
All 6* Rank 3 And Awaken And i got 400 tier 2 Primordial DustWarlockKushalaCosmic Ghost RiderOnslaughtSilkNick FuryPhoton