As someone with more limited space I am big on keeping a feeder that I also enjoy looking at.
I enjoyed making my own colors with the guppies and endlers. Was exciting to see some unique ones showing up in my tank while also feeding the less interesting ones to other fish. They are also the most reliable as far as constantly having small fry on hand. I would call them the objectively best feeders assuming the fry size is fine.
I think the other livebearers are a bit slower than guppies/endlers while also being more prone to eating their own fry. If you prefer platies they seem almost as good as guppies, while swordtails are larger so you would need to devote more space to them if you went that route. Still if you enjoy these species more then I would go for them. The larger mollies seem to be the least prolific of the common livebearers, so I would be hesitant about trying to use them. I have never tried the smaller mollies, so unsure about them.
As for other options many use Convicts, but there are actually several close relatives many of which are just as prolific if those interest you.
The common jewel cichlid is beautiful while also being an equally prolific breeder in my opinion, they are also a bit smaller than Convicts if that is a concern. I am a huge jewel cichlid fan, and I have kept some of the less common ones. Thus far all have been terrifyingly prolific even the wild caught specimens I have had start breeding as soon as they can.
I am sure there are some others. Perhaps Brichardi would work? I have not had them myself, but I know those who keep them always have fry. The one's at the fish store near me also consistently have fry in their tank which seems like a testament to their prolificness. Like I said I have not had them myself though, so maybe they are not consistent?