Most powerful, at most ECL 18, Neutral Evil monster?

[Malistrae]

I have been contemplating turning my level 18 NE wizard into a monster, permanently. I know I am going to sacrifice my god-like versatility, but it's for roleplaying purposes (also, to somewhat mitigate my party's ridiculous effectiveness) . The DM already greenlighted the idea (obviously, it is easier to dm for an ECL 18 monster than a level 18 wizard :D) and said that I can use the Ritual of Vitality from Savage Species. Now, I only have to find a monster with an ECL of 18 at most. However, the DM also placed some restrictions: no undead, only sentient monsters and (usually or always) NE alignment necessary. I don't plan on turning into some kind of a brute, so spell-like abilites and innate spellcasting are preferable.
Based on these criterias, what are your suggestions?

[MilesTiden]

Hmm. White Ethergaunt (Fiend Folio, pg 67) is ECL 16, and fits all of those requirements. Also, you know, 27 Int and 13th level Wizard casting.

[Malistrae]

White Ethergaunts look interesting. The immunity to level 4 or lower arcane spells are nice and enslave looks like a pretty great ability. However, I am not fully convinced about them.
I am also considering arcanaloth (Monster Manual II, pg. 203), since they get spellcasting as a 12th level sorcerer, plus yugoloth traits and several spell-like abilities (like shapechange at will). They don't have a given level adjustment, but it would most likely be +5 or +6, so they just fit in to my ECL limit.

[Jeff the Green]

They don't have a given level adjustment, but it would most likely be +5 or +6, so they just fit in to my ECL limit.

+7, actually. It was given in the 3.5 update booklet.