As @NotHereToPlayGames said, there's no way for us to answer that question for you, but here are the key differences:
New Moon 28, Serpent 52, and IceApe all use the same underlying platform: UXP. The main difference is the user interface. If you've been using Firefox 52.9, Serpent will look the most similar. Serpent also supports a few features not in the others, such as "Web Extension" add-ons and a multiprocess mode. Since you have lots of RAM you may find multiprocess mode helpful.
If you've been using SeaMonkey 2.49, IceApe will look the most similar. Like SeaMonkey, IceApe is actually a suite that includes both a Web browser (IceWeasel) and a mail/news reader (IceDove), so if you download IceApe, you don't need to download IceDove.
New Moon 28 (and 27) have a user interface from an older version of Firefox that you may prefer.
Arctic Fox, New Moon 27 and K-Meleon use an older underlying platform. The main advantage is, these browsers take less RAM/CPU, but it looks like you have plenty, so you'll probably be happier with one of the UXP browsers above.
Finally, there's Serpent 55, derived from an alpha version of Firefox 53. It looks very similar to Serpent 52, supports more/newer "Web Extension" add-ons, but is less compatible with some modern Web sites. You might consider it if you need a specific add-on that requires Firefox 53.