If I can have only 1 metamgic feat for my wizard, I will choose quicken. From my personal experiences, quicken magic is the most important feat that makes a wizard more useful than having a sorcerer in a party. It is related to number of magics you can cast in a limited time and how fast you can cast effective spells as soon as you encounter enemies. Some spells are even useless without a feat of quicken.
First, quicken spell allows you to cast more spells in a short time. In some ways, art of playing a wizard is "moderating" the overall difficulty of a whole quest. In other words, it is not wise to use too much SP for the easy fights that other members can handle with a simple haste. Thus, This feat shows its worth when it comes to a deadly battle. For example, when you encouter 5 monsters including a cleric and a caster, you can finger the caster + web on cleric + mass hold on the rest while a caster without the feat just finger'd the caster and starts to cast next spell. Shortening 1 or 2 secs at the encounter makes bigger diffence to the result. Your party's cleric can save more SP, haste will ba lasted for the next battle. It also reduces the chance of any death as it is operated more effectively as a art of 'moderation'.
Moreover, this feat affects how fast you can react when you encounter the enemies. First-encounter is the most important point of managing aggro. The battle goes completely different by how you manage your first encounter. Charming spells are a good example. These spells help you better if you sucessfully make the mobs to fight each other, without any of your party member getting aggro for few seconds. Casting a fog or firewall before any of your melee friend grabs any aggro makes it easier to handle the battle. This kind of skill does not definitely requires the quicken feat, but it is much more easier and faster with the feat.
Lastly, this feat adds more value to some spells. Especially, touching spells without this feat have much lower chances to success because of the concentration check is needed if you have aggro, or mobs move too fast to be succesfully hitted by the spell if you dont have aggro. Quicken let you cast Otto's dance on cleaving orthons, goul's touch on hobgoblins, electric/chill touch on mobs when you need them. The feat also helps battle against beholders in which you have much lower time-gap available to cast any spell in his range. The faster you can cast, the more chance to cast it.
In conclusion, quicken spell feat is among the most important feats for a wizard in my opinion. It maximizes the output of various spells at the time when you really need it. It also eases your aggro-management in most cases, especially in zerg group which is now very common for most dungeon. What's more, there are some spells and situations require this feat. Dont hesitate to take the feat on your first or 5th level, its very useful.
P.S. I am practicing english writings. Any comments about grammer, choice of words will be very helpful for me