It depends on the length of the feed line. For less that maybe 100 feet, almost any coax would work fine. I think something like RG-58 would work okay for shorter lengths, maybe 50 feet or less. There's no practical need or benefit using '9913' or LMR-400 and such at HF. Losses amounting to less than a dB or two are meaningless at HF unless there are a bunch of them. That's not the case with VHF/UHF, but it is for HF. If you just feel like you should use a 'better' coax, it's your wallet, knock yourself out!
Thanks for all the suggestions. I found some RG8x pretty cheap and I am definately going to keep it under 50'. As I start getting back into HAM I'll probably get a better rig and antenna system. But for now I just hope I can get my antenna tuned!
No, you wren't mistaken about RG-8X being 50 ohms impedance. I should have made it clear that I was talking about RG-6 coax which was suggested by K9WG.
While that 75 ohm coax may provide a nice match between a 'flat top' 1/2 wave dipole and the feed line, you'd still have a 50/75 ohm mismatch at the transmitter. If you 'droop' the legs of that dipole, you lowered that 75 ohm input impedance to something closer to 50 ohms. So, why not take advantage of that and use 50 ohm coax? The -only- redeeming quality about that RG-6 is that it's cheap. Try soldering to it...