This nice guide was taken from a post on Steam Forums:
NyteFire's super easy guide to getting lower pings
Ever make a lobby and it seems that the only people that join are from half way around the world? Ever join a game and everyone else is pinging green yet you're in the red? This guide will solve that problem and also help you get lower pings when you host lobbies.
For starters go to your Steam icon on your taskbar and click on settings, then to downloads, and then set the download location closest to you. Back when l4d1 was out my download center was in China while I'm in Houston, Texas. Doing this will allow you to connect to lobbies which are in your general area and allow players in your general area to connect to your lobbies as well.
Secondly enable your console. Go to options, keyboard settings, advanced, and enable developer console. Doing this will allow you to enter essential console commands.
Now that your console is enabled you have two ways of getting low ping servers. The mm_dedicated_search_maxping and the mm_force_dedicated_server console command. The former can be used to find ANY server that you ping less than or equal to the max ping you use in the command. If you set it to 50 then you will only connect to servers that you ping 50 to or less. The second command is used to connect to a specific server. Using the openserverbrowser command you can view many l4d2 servers. When you find a server that you ping low to you can use that server's IP to connect your lobby to. Copy/paste the server's IP and enter the IP after the mm_force_dedicated_server command.
It would look like this
mm_force_dedicated_server 1.2.3.4:27015
Keep in mind that you have to enter these console commands BEFORE you create a lobby as you can not open your console inside of a lobby.
One more thing. For the mm_dedicated_search_maxping command if you can not find a server using the value that you have specified then I would recommend upping that value as there may not be any servers that you ping that low to.
Hope this helps for the pings now I'll talk about the difference between Official Dedicated and Best Available Dedicated.
Often times when I host lobbies I have some nerd yelling into his mic that Official Dedicated servers ping better then the latter. This is false. A Best Available Dedicated server can ping the same if not better (or worse) then an Official Dedicated and this is why.
There are only a few key difference between the two server filters. Best Available includes both modded, non-vanilla, servers and Official Dedicated on the list. The difference between the two is that one has vanilla commands and the other does not. Both servers can run terribly and both can run amazingly. An "official dedicated" server can be ran outside some kid's house with a terrible upload speed to where he can not support 2 connections or it can be ran inside of a data center with a dedicated upload and download line. The same can be said about a Best Available Dedicated server. If can run from either a house with a bad upload speed or inside of a data center.
Not all Official servers are even run by Valve. I could buy a server from Midnight Gaming and have it set to either Official or not. I could even have a custom MOTD and server banner but it will still be an "official" server.
In summation: Official Dedicated servers do not ping better then other servers as there are a lot of different factors when it comes to running a server. If you just want plain vanilla server options then stick with Official Dedicated. If you want to chance connecting to a server with low ping and the possibility of unwanted server modifications then go ahead and select Best Available. In my month of playing I've only run into one unwanted server while connecting to a Best Available Server.
Hope I cleared up some misconceptions as well as helped some people connect to lower pinging servers
