Hey supercarman,
Welcome to the forum!
Optimal is a relative term, however . . .
The Accucell 6 (50W) is a full featured charger and should be able to charge that pack at just at 1C rate. While you might get away with charging it more aggressively, if the battery isn't designed to take it, it tends to damage the pack. Overall, it's a reliable charger at a good price.
If you want to go higher, you can step up to an 80W model and charge at 1.5C, or a 150W model and charge at 3C, but in either case, I'd advise against it if you want "long battery life" to be a part of your "optimal" goals.
Keep in mind, regardless of which charger you choose, you'll need a power supply capable of delivering at least that amount of wattage to the charger, with a voltage of 11-18V.