Really depends on your budget. In general the more you spend the better quality you get. Obviously I think you should get one of my ARs
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https://www.horizonarmsresearch.com
If you mean AR Pistols - I am a HUGE fan of .300 blackout for a bunch of reasons. I don't really like 9mm ARs (but many people do). 5.56 is fine as long as the barrel is 11.5" or longer (I have a few shorter barrel 5.56 ARs and imho the tradeoffs aren't worth it for most use cases).
If you mean a normal handgun pistol, then you really should try various weapons at a range/friend's house, etc.. Everyone's hand ergonomics are different, and what guns will feel good and more importantly point naturally will vary person to person. Also depends on what you want to do with it, i.e. a full size handgun for open carry/truck/bedside table vs. a sub-compact for concealed carry, etc..
I'm a big 9mm fan for handguns in general.
There are a ton of aftermarket parts for the 870, and it's a great shotgun!
Personally I like my shotguns semi-auto (reduced recoil and better tactical efficiency IMHO) like the Benelli M4 or IWI TS12.
If you want info/advice specific to your needs/situation/etc... feel free to email me too:
devon@horizonarmsresearch.com Happy to help if I can!