I've noticed recently that Taric isn't used as much in competitive play. I was wondering which AD carry works "best" with Taric? Also, what kind of runes/masteries should I be running on him?
Taric works well with Caitlyn, since if you have proper communication between the two players, the enemy can step on a trap, Taric stuns, then Caitlyn drops another trap, which gives Caitlyn about 4 seconds of attacking an enemy without retribution due to her superior range. Vayne can also do well with him, if you manage to condemn an enemy into a wall, and Taric follows up with his own stun. Also, Taric/Graves can practically 100-0 an enemy at level 6 due to their strong burst. For runes, armour marks, gold per 10 seals/quints, and either mana regen or MR glyphs work well on him. Movement speed quints are also an option, since you can get in range to land more stuns.
Taric works well with AD carries that can follow up on his stun with either a huge amount of damage or another form of CC. However, there are several lanes that are seen much more frequently than others:
Urgot (Flash Swap -> Taric Stun -> Jungle gank is almost a guaranteed kill at level 6)
Caitlyn (Taric Stun -> predicted Caitlyn Trap -> Shatter + Piltover is a huge amount of harass)
Taric, like all supports, benefit from GP/5 and defensive stats. Support runes are a lot more flexible than others in terms of preference. Taric does not scale well with AP or AD as a support, so defensive runes are the best option. Mana-regeneration is also an option, but this is lane dependent and playstyle dependent. GP/5 is essential for Quintessences in my opinion.
Taric is a well-rounded support in that he has damage (Shatter and Radiance), hard CC (Dazzle), and sustain (Imbue). Caitlyn and Urgot take great advantage of the CC and damage through Trap/Piltover and Noxian Corrosive Charge/Hyper-Kinetic Position Reverser. If you want a sustained farm lane (see: Kog'Maw, Ashe), don't pick Taric.