Hello! ZEF person here! These are my personal thoughts on the different fleets. (Note: This post will not contain any fleet bashing! If you are on the hunt for drama or dirty laundry this post is not going to satisfy! :P Nor has been approved by the fleet staff. It is strictly my personal post.)
The four that have been mentioned by VelvetSilk are certainly the ones that I recommend whenever someone comes to me and asks for a recommendation.
Storytellers - This is a very large fleet with a ton of activity. If you are looking for somewhere to spread your ultra creative wings, this is the place to go. Good people! I dont know them very well personally, but I've never had problems with them and my interaction with them have been positive.
Argo - I could go on all day about how much I love these people. They are SO nice and the inter-fleet work that they do is amazing. They are officially partnered with the ZEF and have an IC trade lane with them so we visit each other's fleet all of the time! They are a ton of fun ICly and OOCly.
26th - This was our home and we loved it. Although we parted ways because of differences, I still recommend this fleet. They are a great bunch.
There are a lot of rumors and here-say about our departure and like all stories with a lot of emotion, there are a lot of things that get exaggerated. The entire thing was heart breaking for all involved and some emotions ran high that caused things to be said and fingers to be pointed. I could go into detail about the poaching accusations, but I am going to leave it simply: Any that did happen was brought to our attention and dealt with immediately because it was poor behavior and not condoned by the staff. That's all I will say on the matter because this is not the place to air dirty laundry. We have our hands extended to the 26th in friendship, and will not put them down. In fact, some of their members and staff come and visit our base despite the (still confusing) OSB on ban the ZEF, they are always welcome to come visit it. Different tags over our heads can not keep friends apart. Just because we had our differences in leadership styles does not mean that we hold personal grudges.
There are a TON of positives about the 26th, they are a wonderful group of people and have an active and dedicated membership. Although OSB has some negative reputation, there are positives about it as well. It is a safe place to go and visit to meet new folks without trolls. The ban list (at least when I was there) was indeed something only used as a last resort when a player did not correct poor OOC behavior after warnings.
The PG13 rating is something that is a positive for some people. It allows for family friendly style stories and open membership for all ages!
I highly encourage you to take a look at their site and see if it is right for you.
The Zenas Expeditionary Force - This is my home, my heart, and the place that possesses my thoughts for most of the day.
OOC Community - If I had to pick one thing that I loved the most about this fleet, it would be the people. We are a close group OOCly and genuinely care about one another. When one of our players died, we sat in mumble and just remembered her fondly, some with stories, some with tears. There have been OOC friendships and even romantic relationships that have been formed. And I am excited to say that about 10 of us are going to be at Star Trek Vegas and some are going to be rocking our Fleet Odyssey uniform!
Staff/Story Council - The Senior Officers and Story Council are tireless. Seriously these people are machines... but loveable, cuddley ones! The Senior Officers all hold equal rank, they debate and vote on issues and policy to keep things fair and wonderful. The SC, are the GMs and manage the story. They do amazing work, every season finale I sit back and go "Wow, how do they top that next season?" And yet every single time they do.
It is an adult fleet - This is not to say "Adult", but we do have a 17+ policy. After previous experiences with minors we realized that we were unable to tell the stories with the freedom of not worrying about someone's parent frowning over their shoulders. This also -greatly- reduces fleet drama which we absolutely despise. Yeah, it happens, but we get a handle on it quickly. We are an adult fleet and we expect members to act like adults. This is something very important to me and not so much to others.
Extra "Curricular" - You might have seen our fleet videos? Our cosplay talent? How about the epic stories or personal logs our members write? Have you seen the screenshot gallery? It might make me the biggest nerd every, but I am a genuine fan of our fleet. Most of that stuff is public, even if you aren't looking to join us, check it out. It all makes the Trekkie in me so damn happy.
IC Talent - I talked about the OOC, but let me touch on the IC. I just love the characters that make up this fleet. They feel so real and believable. I'm constantly watching one character or another because Im so interested in their lives.
That's my two cents! The ZEF staff can be contacted on the site for official statements. They are nice and answer questions quickly!
I hope you find a good fit! All four fleets are definitely worth checking out!