I would stick with RG8 size. I know a lot of guys will chime in and suggest LMR400, yep its good stuff, but spendy compared to other options, not to mention its stiffer than you know what and you can't make sharp turns with it.
I used to use 9913 but i don't like the hollow air core types unless your really careful about sealing it up, otherwise you end up with a water pipe.
I use a lot of Jetstream JT2015F, its the flexible variant of their RG8. They also make other versions of it as well. It's affordable, comparable specs to 9913 type coax and much more flexible with its stranded center.
Jetstream
Click on the pictures for the specs.
Either way, no matter what you do, at UHF try for the larger RG8 lower loss variants. Not some cheap RG-8X, RG-58, etc..