I can't tell you the worst pick right now, but I think it is currently the Titan, the 780ti is comparable and is much cheaper, I don't get the Titan's purpose. I may be wrong though.
For the best, I usually go with the 7850 after rebate for $100, but I don't count rebates or limited-time sales so I would probably go the R9 270/270X for about $150-200.