From my Exp, anything with better odds. Dream packs you can get extremely lucky and pull a high card, but those are usually a waste of coins. And unless you plan to spend a lot, its not worth it. (Dreak pack odds get better the more you pull, as there is a set number of cards in the pack)
Step up packs seem to be the best IMO. Or Set packs (packs with a set group of cards in each pull)
Set packs, like Weapon packs from Weapons of choice event are not a bad choice, since usually every pack has something decent in terms of EU.
Step up packs don't guarentee any EU, however i have had the most luck with those.
All in all, it depends on how much you feel like spending. Try to save for the discounted packs, those are always better values. And try to figh tthe urge to just go for any 9.
Try to aim for Special 9s
All of these were good packs to pull because the 9s were unique.
Amelise was the first 25 DC 9
Kasumi was the first A speed 9
Liuth is the Only A speed Missile
Libitina was the first B speed Magic
You can always get lucky with any pack, but Step ups only have a pool of 50 cards to draw from. So even if you don't get the 9, you will likely get a slew of 7s and 8s. (and 5 and 6 feeders)
Hope this helps~