If you want to do a fish in cycle you really have a lot of options. People always suggest Guppies. They are a great, hardy fish that do well through a cycle that's for sure. The color variations are also vast.
Personally, because you haven't gotten any fish yet, I would suggest you think about what you want in your tank long term. Decide what stock you want with your tank and then see if there are any fish in there that are hardy enough to survive a fish in cycle. While you can start a cycle with guppies it's always best to start with a fish you do plan to keep long term and will fit into your end idea. If none of the fish you want to keep could survive a cycle than I would recommend a fish less cycle. Fish in cycles can be some work too, so be sure you are ready to take that on. Fish less can be a pain because you look at this empty tank for weeks before you get to put something in it but it certainly has it's pros as well.
50 gallon really does give you a lot of options. I'm actually getting one today!
Platys are another hardy live bearer and come in many colors and patterns.
I would also suggest that you go into your pet store to look, just look no buying
(it's hard sometimes) and see what they offer and what interests you. Then make a list, bring it home and research these fish to see which suit your tank and if they will be good tank mates. Nothing sucks like seeing a fish online and going to the store to realize they don't sell it, or do but it's far more money than you hoped to spend. Or you have a worker tell you something will work in your tank and it doesnt, which is a common thing, sadly.