Hmmm... I would say Hellsing Ultimate. It's an series of OVA's from the anime Hellsing, the only difference is that the OVA's follows the manga completely.
There are now 9 episodes, all 50 minutes long. the last episode will come out around the end of august.
This anime is the sickest, most bloody, killing/murdering anime I've ever seen and it actually has a pretty good story too =)
Not to mention the animation, which is definitely an 11 out of 10!
Be sure to watch it, it will blow your mind =)!!!
If it's for gore, I'd say "Deadman Wonderland". If were going for action, i think "BTHOOOOOM!" will be the most action filled one when it develops more episodes. They just started and I'm excited to see the manga develop into an anime.