As previously stated, Talon is an extremely situational pick in ranked. Don't even think about playing him if you don't already have a solid AP champ on the team going somewhere other than mid.
Champions that synergize well with Talon:
Be very careful with
Elise, as that will leave you with two exceptionally squishy champs, not including your ADC. It can still work if you have your jungler and support have a lot of tankiness/CC. If you've got a squishy top, make sure your AD carry has a lot of mobility.
The most important thing to have on a team with Talon besides a tanky AP champ is crowd control.
Diana is my favorite champ to have alongside
Talon because she's tanky, deals a lot of damage, and has good CC. Use a jungler such as
Amumu. All of them are tanky AP champs with hard CC and lots of utility.
Lee Sin and
Shyvana are bad ideas because they lack hard CC (excluding Lee's ult). Some good ideas for supports are
Alistar. Stay away from
Soraka. Sona can still be an okay choice because of her ult, but her squishiness and general lack of CC hold her back. Keep in mind, this is solely in regards to team comps for Talon. Sona, Lee Sin, Shyvana, and Soraka are all excellent champions and can fit in with many good team comps, but they simply don't synergize well with Talon.
So, what do we need in regards to team comp?
Lots of CC
If you're planning on playing Talon, your team needs to essentially be built around him, while still maintaining its integrity as a viable comp. Of course, all of this doesn't even include what the enemy team is picking.
Pick Talon when the enemy team has...
A lot of squishies
A lot of AP
Don't pick Talon when the enemy team has...
A toooo tanky mid
More than two tanky champions
Shen or Malphite
As you can probably tell, the big issue with Talon is that a lot of conditions have to be met in order for him to be a viable pick in ranked. However, with the right team comp on both sides, Talon can be an incredibly effective assassin and will carry the team under the right circumstances.
If you're unsure about whether or not you should play Talon, then don't.
It's far too easy to get counterpicked and shut down if you don't know what you're doing. Unless you've got a lot of practice with Talon and are confident in your abilities to outplay your enemy and adapt to a multitude of situations, I would recommend sticking to normals with him.