Need to buy long (75ft) coaxial for cable internet modem. What's the best kind?

RG6 coax, Cat5 ethernet, and Wifi will all work, take your pick. i doubt that you would lose any datarate with the wireless, the bottleneck is further up the line in the Comcast system. for a wired network the coax would be the most rugged, especially if you have to run under doors, and keeps the modem by the computer, one less thing for baby to play with.