Assuming you can pick 4 levels of Master of the Unseen Hand PrC, anything works and you don't even care for your BaB anymore.
This means 2 things:
1) A TK ghost is an awesome dipper, so don't look for THE best base class.
As an example, look for marshal 1, hexblade 2 and paladin 2 (of slaughter or tyranny if you also take hexblade, which is restricted to any nongood) for awesome cha-based dips. Take them all and you'll have (assuming motivate dexterity) awesome initiative and awesome saving trows (keep in mind that, as you are incorporeal, basically almost everything that can hit you is a spell or spell-like effect). Hexblade 3 gives you mettle, which is awesome as hell with your already awesome saving trows. Add ring of evasion and laugh. I'm not sure if you can add SR somehow to have a great non-TO defensive build.
There are a lot of awesome dips. The usual Cleric 1 for domain powers is one.
2) If you really don't want dips, just pick a cha-based spellcasting class. Sorcerer will work. I'm almost positive the best choice is found here as per the usual law of optimization: a spellcaster does everything better. This, or something psionics (I really don't know psionics, but I bet if the answer is not spellcaster, then it's them).