There are two main factors on coaxial cable for a receive application. The first and primary item is loss for the highest frequency you plan to use based on your cable length. Since your run is 30 meters you can use the common 100 foot loss numbers. Be aware that as the frequency goes up the loss per foot (or meter) nearly always goes up so a cable that works great on 40 MHz may be really horrible when you change to an 800 MHz frequency range system.
The second item is impedance of the system you have. Generally most scanners and antennas designed for them are based on a 50 ohm design. Don't get too hung up on the impedance though. Since a scanner is a very wide band design and impedance is based on one (or more) frequencies in the design specs it really isn't that important that things match exactly. Basically what I'm saying here is that even though your scanner and antenna specify a 50 ohm impedance you won't break anything by using 75 ohm cable if otherwise that choice is warranted (often it's less expensive, easier to find, and easier to install). If you transmit using that cable it does really matter that the impedance matches!
Two additional comments, one real and one somewhat a joke. First the snarky comment. Since you asked for the "best coaxial cable" you're really limited to a very expensive and hard to install rigid coaxial cable like they use in the very tall TV transmission towers. They are 6+ inch copper tubes with an additional copper tube inside, weigh a few hundred pounds per section, must be mounted using a spring tensioning system (since copper expands and contracts at different rates than the steel used for the towers) and the typical installation has a six figure cost.
OK, now that the snarky comment is done here's the one that you can actually use. The connectors on each end of the cable should match what it's designed to connect to. It's perfectly OK to have different connector types on the two ends of the coax. For example, if your antenna requires a PL-259 connector and your scanner requires a BNC, that's how you should request your cable be made. That said, there are reasons why this isn't always possible. For example, not all connectors exist for all types of cable. This is especially true if your scanner uses an SMA connector. While it's best to match each end directly, you may be forced to use one or more adapters. If you must, you must but keep them limited to as few as possible. Adapters = loss and you really want as little loss as is practical. Now back to the SMA connector. If you must join a thick and heavy cable to an SMA connector you should really use a short jumper. A jumper will make it easier to find the correct connector for your coax and the jumper is much thinner and more flexible cable so there will be less stress on the SMA connector (so it won't break nearly as often).
I'm sure that others will provide more detailed answers, often like "I use RG-xxx and it works great!". OK, that's a good opinion, but may or may not fit your situation. That coax may work great on low frequencies and/or for short run lengths but will it work for your installation or length of cable. You did indicate your cable run length and that's great (many times folks don't provide that), but there are several important details needed for a proper answer. Look back at my second sentence and you'll find a critical detail you didn't mention. That detail is what's the highest frequency range you'll actually be listening to. Be aware that it really doesn't matter what range your scanner will receive, if you never listen to the highest band it covers it will needlessly restrict the answers to larger and more expensive cables. Also it's important to indicate how you plan to install your new cable. Some cable types have a pretty simple installation while others are very stiff and a real pain to install (but their gain numbers are great!) so for the best answer more details provided will generally produce the most balanced answers based on what you provide. For example, "I need to listen to up to 960 MHz signals, it must be very easy to install, and I have an unlimited budget." will give you a totally different answer than "I listen to nothing higher than 170 MHz, I can handle a moderate install, but easier is better, and I can only spend no more than $50 on the coax and connectors."
Good luck with your new install!
