my $0.02...

lvl 15? so that's six feats total + any bonus for race etc yuh?

Improved Initiative is always a good choice, especially for sorc's who dump dex.

i like Eschew Materials as a feat, never have to worry about being captured & stripped etc.

Spell Focus if there's a school of magic you use a lot.

Skill Focus, if there's a skill you really don't ever want to fail a check (concentration is a good one)

Heighten spell is always nice if you really need to bump up a spell DC.

silent/still spell for sneaky casting.

Arcane Thesis, if you want masses of metamagic cheese.

Quicken Spell: only if you're a metamagic specialist.