Vladamir I would say is the best carry of the three. It may not seem like it, but his E with 4 stacks makes his sustain in teamfights immense and his AoE damage superb. The downsides are that you will not get any kills in lanes, so laning will become a little boring unless you can poke your opponent out, or simply outsustain them.
Lissandra I think is the most fun and very safe from ganks. Though she cannot withstand a sudden all in, so if you make a single mistake she's done. Her damage falls off later on especially because enemies will not purposely be close enough to hit Q (which has a SMALL initial range), but at that point your goal is to initiate, assassinate or peel.
Renekton, I'm not a fan of. I've seen very good ones, and he is the only real tank option of the three. But if you can't get kills in lane, he simply becomes that guy who can catch someone, stun them, and then become a sponge.