Not Updated For Current Season
I decided that I should make a contribution to this site instead of just looking at other peoples builds and therefore created this build for Lux. I chose Lux because I think she is an amazing champion for people new to the game to play and continues to be really exciting for more advanced players as well.
For me, choosing Lux's runes was an easy decision to make. I think that magic pen is really important for Lux so I simply set all my marks and quintesences as magic pen straight off. With my glyphs I find it useful to have a bit of ability power at the start of the game to help with first blood; however I prefer to use glyph of force to give me more ability power by the time I reach level 18, this helps me to achieve the main focus of my build - getting tonnes of ability power to create the highest possible amount of damage. Following the theme of giving Lux the highest possible ability power I picked secondary tier three runes that continue to boost my ability power. If anyone has any other views on the seals I would be more than happy to test it out and change my build accordingly.
For my masteries I chose 9 / 0 / 21 considering that I wanted to grab archaic knowledge in offense and then build as much as possible into utility - I went in both trees to grab both of the cooldowns. I use Lux as a support character when it comes to her light binding (Q) prismatic barrier (W) and lucent singularity (E) moves during the early stages of a game, however as the game progresses I find that Lux's light binding and lucent singularity moves become important when I need to deal out as much damage as possible.
In the offensive tree I grab cripple as I like to exhaust champions - slowing them down and decreasing their armour and magic resistances. This favours not only me but it helps the rest of my team out as well. I like to slow enemy champions and use Lux's light binding to lock them in place and either inflict my own damage or I can use lucent singularity to continue slowing them down after the exhaust has worn off, giving a team mate more time to bring down an enemy champion.
Essentially the items I pick are all about heightening my ability power, however at the start of a game I go for a sapphire crystal and I also grab a mana potion. As soon as the gold rolls in my next move is to get Lux a blasting wand for her ability power and then I'd get the boots for some increased movement speed.
As a match progresses I'll pick catalyst the protector and upgrade to the rod of ages ready for it's stats to increase - it's pretty handy. Then it's time to upgrade my boots to sorcerer's shoes. However if I'm against a really fast team, with a tonne of attack speed, I consider whether it'd be better to grab the boots of lucidity and get better cooldowns. I then save my gold for a bit so I can get the needlessly large rod and a blasting wand. I can then upgrade them and get rabadon's deathcap which'll put my ability power up to around about 200.
My next step is to build the archangel's staff, so I'll grab the components - another blasting wand and also a tear of the goddess. This staff is a fantastic item once you've built up your mana pool because it'll increase your ability power with 3% of your total mana. This is the point in the game where Lux can really start to show off her dps side, especially with the use of the Finales Funkeln move.
The next step I'd take is to build Lux Lichbane. It's a really great item for her because it'll give you another 80 ability power, 350 mana (which'll go towards archangel's 3% ability power increase!) 30 magic resist (useful in some circumstances) 7 for Lux's movement speed and then my favourite bonus - a 100% chance that when an ability is used, your next physical attack will deal an additional 100% of your total ability power. Lichbane works well with Lux's passive as well; if you use light binding to lock an enemy champion into place Lux's passive - Illumination - will play its part in increasing damage and teamed with Lichbane will cause massive damage.
The last item I build is always Zhonya's Hourglass which'll grant Lux another 100 ability power and also 50 armour (not overly important but can still be slightly valuable). The hourglass also grants Lux invincibility for 2 seconds which has come in handy for me on quite a few occasions. Lux is often seen as an easy target because some people assume that she's a fairly squishy character. If this is the case then she is likely to be ganked by the opposing team and if this happens, a good team will come running to help you; so in those vital few seconds before your team arrive and take the heat off of you, Zhonya's Hourglass mixed with your Prismatic Barrier can really prevent you from feeding an enemy champion.
To me the skill sequence is totally up to the player, but I would always start with Lux's light binding (Q) move first and then grab her lucent singularity (E) then prismatic barrier (W) move so you have all the three starting moves, I prefer to grab her light binding as fast as possible to do damage to the enemy champions, I see her lucent singularity a great move to slow champions, but i use it more to get the minions towards middle game as once the ability power has started stacking higher the minions die after one hit, Lux's finales funkeln (R) move is grabbed every time available at level 6 / 11 / 16 at the end of the game you will notice how much damage she can do, it works well in a combo of using light binding auto attack and then finales funkeln to finish the enemy champion off, her prismatic barrier move is grabbed more for support to stop that little bit of damage until ability power has risen at what point it can help in the worse of situations and it can also help your team of champions.
The summoner spells can differentiate depending on how you would rather play, I like to use exhaust and ghost with Lux, as I can chase with ghost and slow with exhaust going in masteries for both.
To summarise the end of my build and guide for Lux I hope that everyone who reads it has enjoyed reading and I hope that it has helped any Lux players out there to do nice damage and own the lanes, if I get a good rating or decent comments I will look into making another guide in time.