To judge what type of ranged AD carry works best with Lulu, we need to look at each of Lulu's skills to determine what type of playing style each skill will benefit.
Glitterlance(Q): (Active): Lulu and Pix each fire a piercing bolt dealing magic damage, slowing enemies hit by 80% for a short duration.
This slow decays over time. An enemy can only be damaged by one bolt.
Glitterlance is inherently a very offensive spell. It has a long range, can hit enemy champions through minion waves and slows them for a whopping 80%. That is the highest percentage of slow any Champion can achieve at level 1. This will of course lend itself to match well with a ranged AD carry that has a very aggressive play style that can hop into the range to attack the enemy(like Graves's Quickdraw, Tristana's Rocket Jump). It can however be used defensively also as you are running away.
(Active) – On ally cast: Target ally gains 35% movement
speed and bonus ability power for 5 seconds.
(Active) – On enemy cast: Polymorphs an enemy champion for a short
duration, disabling their ability to attack or cast spells and
reducing their base movement speed by 60.
If Whimsey is cast on an ally, they gain 35% movement speed and gain 20-60 AP. This may seem like it would be beneficial to choose an ranged AD champion with AP ratios but generally, these champions AP ratios are not that great. I find that in a fight, it is way more beneficial to a cast Whimsey on the enemy champion to Polymorph the enemy into a cute impotent creature that cannot harm your ally. This skill is great during the harass phase. Casting Whimsey on the enemy champion will allow for a very one side harass for your AD carry. This means it is better to have a AD carry who can do a quick burst of damage then back away when Whimsey's duration runs out. Inevitably however, there will come a time when the ranged AD carries decides to get into a full on fight. Usually Whimsey on the enemy will give your ranged AD carry a huge advantage over the enemy.
Help, Pix! (E):
(Active) – On ally cast: Commands Pix to jump to an
ally and then follow and aid their attacks instead of Lulu's for 6
seconds. Upon arrival, Pix shields them for 6 seconds.
(Active) – On enemy cast: Pix deals magic damage to target enemy unit.
Pix then follows and grants vision of them for 6 seconds.
Generally Help, Pix! should be cast on an ally for the shield and the bonus damage from Pix. Although Lulu won't be able to damage the enemy, the bonus damage from casting Help, Pix! on the ranged AD carry should make up for that though. Ranged AD carries that benefit most from Pix is a champion who has a built in attack speed increase skill. The Pix aid will only last 5 second so the attack speed skill should be used as soon as Help, Pix! is casted. Grave's Quickdraw increases his attack speed by 40-80% for 4 seconds and Tristana can increase her attack speed 30-90% for 7 whole second using Rapid Fire.
The only situation in which Help,Pix! might be beneficial to cast on the enemy is if you and your AD carry is chasing someone through the jungle. Help, Pix! will you a very useful vision of the enemy.
Wild Growth (R): (Active): Lulu enlarges her ally, knocking enemies
away from them. For 7 seconds, her ally gains bonus health and slows
nearby enemies for 1 second.
Wild Growth generally do not benefit one ranged AD champion over another. It is a great skill used to save your AD carry in the event you guys get ganked. It will knock the enemy back and give you AD carry extra health to make it back to the turret. The slow is also useful to escaping. But because the range of the slow around the ranged AD carry is generally shorter than the range of the carry's attack, the slow is not very well utilized on a ranged AD carry.