It's hard to pick between Gekidou and Doubt & Trust. Both are quite good. Doubt & Trust is probably the catchiest and hypest opening of the bunch, begun to air when the series started to reach to its best part (Fallen One + Edo) and has dynamic and cool visuals, while Gekidou (being the last opening of the original anime) has a nice yet sad feeling of "final battle" (I think its the saddest sounding opening of the series), has great visuals too, its chorus is incredibly catchy and it aired during the best bit of D. Gray Man (Noah's Ark + Black Order Invasion). I think I slightly favour Gekidou more cause the feeling of "epic final fight".
Innocent Sorrow is great too. I think it set perfectly the mood of the anime, it's catchy and it has good visuals as well. Brightdown was nice, the song is dark and epic and the visuals of the song were promising, but, since it came in the part with the most filler of the series, the end result was kinda disappointing. I consider Key-Bring it on my destiny- the worst opening out of the five, it is the most generic-sounding and the visuals didn't hyped me much, but I still liked it more than your average seasonal anime opening, I will give it that.
At the end it would go like this:
Gekidou >= Doubt & Trust > Innocent Sorrow > Brightdown > Key-Bring it on my destiny-
