I've been a huge fan of BB since the beginning. It's the GOAT, for me.
I enjoyed GoT, though I finally admit it really start to click for me at the end of Season 3, and Season 4 so far has been amazing. I'd say it'll take a few more seasons for me to admit that it's overtaken BB.
That said, a poor series finale, or poor final season, of any show can soil the reputation of the entire run. If GoT overlaps GRRM's books, they'd just have a broad summary of where to go... that's very concerning. I doubt it'll turn into Dexter, or Lost, but it'll be a big dent in the overall quality of the show, as compared to BB that had a perfect run, from pilot to finale.