Egoist has Hustle, Metamorphosis, Fission, Fusion, and Greater Metamorphosis. Those are all powers which I consider invaluable in any psionic nova, but can be worked around via high level Expanded Knowledge choices.
Kineticist has Control Body, the basis for a lot of Gishes, and a lot of powers that DD loves, but otherwise kind of sucks (why not just augment some general psion stuff instead?).
Shaper has Astral Combat, Psionic Minor Creation (WHICH IS AWESOME!), Fabricate, Astral Seed, Genesis. Crystalize never works (well, thats an exageration, but not much of one). These are all great powers for long-term power, but they aren't so great at Novas.
Nomad has been covered; its OK, but IMO the XPH powers are all easily imitated by proper magic items and normal spellcasters. Evade Attack is freakin' awesome.
Telepath has Psionic Charm, Brain Lock, Crisis of Breath, Dominate, Schism, Dominate, Metaconcert, Mind Switch, Crisis of Life, True Mind Switch, and Psychic Chirurgery. All these are about as awesome as powers come, all at the level which they become active.
Seer has Second Chance, Fate Link, Remote Viewing, Fate of One, Metafaculty. Thats all well and good, but its not filled with a lot of really fun stuff for a varied experience.
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So it all depends on what you're building for. For a character that can "fit in" in any group, I think Telepath has the best powers not easily duplicated by spells.
If you're trying to Gish, I'd say Nomad or Egoist is the way to go.
The best Novas come from combinations of Schism, Fusion, Fission, Assimilate, Time Hop, Feat Leech, Power Leech, and the Leadership feat and/or Thrallherd.
All that said, I'd say either Egoist or Telepath gives the most unique stuff, followed by Shaper. The rest of the abilities are easily duplicated by non-psionic items.