I like most of 'em, and which ones I'd pick depends on what kind of caster I'm building.
If my PC depends on buffs or spells with significant durations, I want Extend spell.
If I'm concerned about #dice I roll and their totals, I want Empower and Maximize.
If I'm an elemental caster, I want Energy Substitution and Energy Admixture.
If I'm playing a guy who specializes in ranged touch spells, I'll want Reach Spell to expand my repertoire...and possibly Split Ray or Twin Spell.
If I'm a martial or subtle spellcaster...one who may find himself alone, grappled or otherwise in melee, Still Spell and Silent Spell are high on my list.
And any of my builds may include a Reserve (or Heritage/Bloodline) feat.