It occurs to me that one thing would change my answer: I think there's a distinct possibility that (if tech allows) the game may be on double XP for a while if it comes back online. At that point I'm sure I'd create several characters for which I want to reserve names, but the one I'd be likely to actually play first would be an Elec/Shield scrapper. It's one of my old 50s that I'd quite like to have back at max level, but I'm not so sure I want to go through the levelling process again.
The four I mentioned above, though, I definitely want the experience, and I wouldn't want to cut it short by doing it through double XP. Actually, when the game was live I tended to play 50s on double XP weekends. I tried levelling a character through it and found that it went so fast that I felt little attachment to it. Just went by too fast for me to feel invested.