This is ALL up to you. I can give more clarification though.
(This is not a best to worst list, simply explains the Regiment's in more detail.)
Shock
I am personaly in shock so my view may have a tad bit of bias (I will do my best to keep it Un-Bias'd) The Shock are a ok choice for beginers to the server, we are open to help anyone who wants to join! That being said, having more in-depth knowlege of RP situations makes it easier. EA, Capturing POW's or searching ships.
69th Medical Corps
This Regiment is a bit understaffed, but that does not make it any less important! If you decide to join this Regiment, you will get a lot of people saying "Thank you" and people screaming "MEDICCC" I would reccomend this for new players due to the chance to learn light and heavy RP, plus who doesn't like medics.
501st Legion
The 501st are a nice Regiment everyone I have ever met from there are nice and polite, that being said, I dont have much experence with this Regiment, But it does host tryouts Offen!
Engineering Corps
The 31st Love their explosives, and there are a lot of newer players there. I like their commanding offcers tho. This is a good one for new and experenced players alike.
Inquisitors
Not reccomended unless you already have a paid rank.
Navy
ONLY for experenced players. Still, good choice if you have tons of Server Time.
SC
Flying in gmod is a bad time, thats all im gonna say.
Commandos
Pay to get in, you get jetpacks though, so thats a plus.
Purge
Very nice people, I would reccomend it. Cant say I know too much though, ask in ooc if you want to know more.
Royal Guard
Uhh, if you want to pay to follow someone around all day, thats your choice.
IF
Please only join if you have a LOT of server Experence, their people are are very nice though!
TY for reading!