Calling=Warrior, Rogue, Cleric, Shaman
Souls=Available skill sets.
Build=Combination of souls
All callings will have entirely valid solo builds comprised of several souls. So... there will be no "ultimate" calling. Also, when combining souls, you are still limited to the available skill points. So you will always be striving to optimize survivability vs DPS, which will shift based on different encounters. The game will force you to switch setups on the fly, for example, from a single target to an AoE spec.
Also, every soul brings something different to the table and it will be important to combine souls which complement each other. For example, inquisitor stuns and slows and is good at ranged casting, whereas shamans are melee based, so the two don't complement each other.
Hopefully, all callings will be able be able to solo effectively. However I suspect there will be certain builds more effective than others, but the nice thing is you can swap into it whenever you want and still have 3 more builds for groups/raids/PvP/whatever you want.
Currently playing: EQ2(AB)