You mentioned the word 'flipping' in your title, and flipping infers 'speed and ease of transaction', so I will respond to that word and say that Godaddy has been by far the best registrar for my domain flipping. They are not my favorite registrar (in fact, I have a lot of complaints/ issues about them), actually only one thing really pushes them into my top choice:
They are the largest and best-known registrar in the world to end users.
When I flip domains (flipping technically means selling quickly, shortly after you buy or reg the domain), it is all about making things easy (and understandable!) to the end user buyer. Having to say something like "this domain is still in the 60 day transfer lock, you'll have to wait 52 more days until that is lifted before I can transfer it to your registrar"... well, you might as well just hit the buyer over the head with a stick and cancel the deal. Since almost every end user buyer I've sold to has a Godaddy account, and since a Godaddy-regged domain qualifies for Afternic Fast Transfer and shows up on dozens of other registrars when people search those for identical/similar terms, it's simply the easiest bet for flipping to end users.
If you're flipping to other domainers, registrar doesn't matter quite as much; most of us have accounts at many different registrars (I currently stick to four, but have had accounts at another dozen or so registrars over the years). Pick any decent registrar and go for it. Avoid the worst ones, as they make pushes, transfers etc very difficult, and also your domain security and tech help at the poor registrars can be in serious question. You can read about most well-known registrars (the best and the worst) in the registrar reviews area here.
In short, and in my experience:
Best domain registrar for flipping to end users: Godaddy (not the best registrar, just the best for flipping to end users because most end users have an account there)
For flipping to other resellers: registrar doesn't matter, as long as it's a decent one. There are a few registrars that are so bad, we won't even buy a domain from someone if it means trying to transfer their domain from that registrar; we'll insist they transfer it out themselves, to their account at a better registrar, and then transfer it to us from that second registrar.
Welcome to Namepros. Do a lot of reading and studying, before you dive into domain investing/flipping. It's easy to lose all your money right in the beginning... and the beginning is when you have the least knowledge and make the most mistakes. Save your money until you have a better amount of knowledge and some kind of plan for what kinds of names to invest in and how best to possibly sell them.
Listen to everything we say, then make your own decisions. I gave you my own experience above, but it may differ greatly from others' experiences, so listen to everyone and pick out what seems to make sense and maybe work for you.
Good luck!