Note: Solars are Epic creatures, and so will any PC's they're compared to be. I am, however, ignoring epic spells here; partially because they can turn any character into a literal god, and partially because both the solar and the PC's they're compared to can get it anyway.
Solars, arguably the most powerful creatures in the Monster Manual. They laugh at balors. They eat pit fiends for breakfast. They annihilate nations with a word (I'm only slightly exaggerating here). Of course there's someone out there who wants to play one.
Despite its fearsome reputation, though, solars are just planetars on steroids (a lot of steroids). The only difference that doesn't amount to 'bigger numbers' is the solar's wider range of spell-like abilities, including Wish 1/day. Totally not problematic...
The question: when do more spell slots and class features start to outweigh that giant heap of spell-likes and racial abilities? I'm inclined towards a LA of +2, which would put the solar 4 levels behind a cleric in terms of casting. This might seem harsh, but remember that the cleric only has extra slots, no extra spell levels: even Epic Spellcasting is immediately available to solars.
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