I have lots of characters, lots of 50s, but only one has gotten significant post-50 play (something like 70 vet levels). I'm assuming that you are looking for something that you can play well past 50, mainly in team settings. Some thoughts:
1. Choose a style that you enjoy. Melee/ranged/support/whatever. If you love blasting, don't make a tank, even if you think it's "better" in the long run.
2. Choose animations you enjoy. You are going to be seeing a LOT of them. There are a bunch of powers that I just refuse to take because I hate the way they look.
3. Here's where it gets opinionated. Play solo and regular XP to your 30s. It's so easy to get to 50 in a few hours that lots of people will bow right past this. But to me, it's important to know your sets. Which work great at level 10 and are useless at 30? Here's also where you figure out which animations you like or hate. I don't recommend doing something like Posi 1, for example, until you are in your 30s because you can level a few times in there and you'll miss some of that learning experience.
4. If it gets boring, move on. I have a bunch of alts in their 40s that I've just stopped playing because they don't currently interest me. Sooner or later, I'll probably take them to 50, but as long as I have more interesting things to do, I'm gonna do those things first.