I resigned from my first job after 3 weeks as a new grad, did not feel safe. Being let go during orientation just means you were not the right fit for that job or unit and I would also bring to question the training being offerred to you which lead to mistakes. People are quick to blame the RN and not the process, If this is the "culture" in that facility I would look elsewhere. What was your patient ratios? Sounds like too many red flags. Being let go there should be no harmful repurcussions to your future as a RN.
On an observation unit you will not be good fit for a new grad, in my opinion. As the population changes and stay is usually less than 48 hours, fast paced, where they are "observed and monitored" to either be discharged or admitted and you won't really "learn" a specific patient population, that could be confusing for a new grad.
A med surg floor would be best for a new grad, the type of med surg floor is up to your interests. I work on a cardio-vascular floor, lots of AAA's and bypass surgeries post op, we also had ENT at one point lots of trachs and suctioning and high risk aspirations but now we got transplant patients. Ortho, would be a literal "pain" think broken bones, fast paced and giving lots of pain medications. A strictly medical floor is more slower paced , not many people going for surgeries such as A hospitalist floor or Family medicine floor is a good learning back ground then you can leave after 1 year if you hate it. Browse around and identify your interests first, despite and pressure you got to start working.
After 6 months at my hospital you are able to float and pick up extra shifts on other medical surg floors so you can find what you really like. I dont particularly like med-surg but the people I work with makes the different and I been at it over 3 years on same floor.