While many people have different philosophies when it comes to pack buying, I personally believe that Mega New Blood packs are the best to buy, especially on the day of new releases.
While Titanium and Armageddon packs offer a better ratio of credits to cards, they are pulling from all cards within the clans that are chosen. Also, since most of the cards in the card pool have been released for a while, their prices are relatively stable.
With New Blood and Mega New Blood packs, you do not have a choice of what clans the cards come from, but the card pool is much smaller. In addition, the New Blood cards are very new and usually fluctuate in price by a lot, which can result in large profits.
In the end, it comes down to your own personal philosophy and experiences, but Mega New Bloods are always a decent option.