Yes, we know you asked about connecting with scada. But the information you've given us doesn't tell us what PLC you're wanting to connect to scada.
Your first post gave conflicting information -- was it a SLC or a PLC. A 5/20B is a PLC5, not a SLC.
Even now, we don't have enough information -- now we know you have a SLC500, but WHICH SLC500. If it's truly a SLC500, that is an OLD brick-style unit, which I would almost bet the farm you don't have.
Most people say SLC500 as a generic term for the SLC platform of AB PLC's. Different models of SLC (SLC500, SLC 5/01, SLC 5/02, SLC 5/03, SLC 5/04, SLC 5/05) require different platforms of communication (DF1, DH485, DH+, ethernet). To tell you how to communicate with it, we need to know EXACTLY which SLC500 you have.
So, give us all the information you have about your PLC and we can give you the information you seek. Give us the model number for your PLC.
Then, as far as a suitable SCADA... you will get many, many different suggestions that will all work. It's kinda like trying to answer the question "What's a suitable car?" with no further specifications or requirements being given.
And to answer the next obvious question (Which one is best?) will purely be a matter of preference. What do you intend to do with it? Are you going to be doing any data collection or report generation? Is it for a single-node or a multi-node system? If it's multi-node, do you want a server-client or a peer-peer system? What SCADA products are supported in your area? What are you already using in your location? Do you have experience with any particular SCADA?