Hi. And what systems work best for play by post?
There are, as can be seen, many schools of thought, including Old School Revival (aka OSR). You are as likely to encounter games that are "rules-crunchy" as you are to find "rules-light.
Everyone has their personal preferences, so I won't be ashamed of mine.
For SF games, I prefer Classic Traveller, aided and abetted by Traveller Plus. Task-oriented mechanics that are, at core, dead simple: meet or exceed a difficulty rating with your task throw and you have succeeded. Average task difficulty is 8 or greater (shown in rules as 8+) on a throw of 2d6. I use this combination for one of my games that I'm re-tooling.
For Fantasy gaming, I'm fond of The Black Hack and it's Tekumel variant, The Petal Hack. These have the virtue of being simple, fast-paced systems and of being available at no cost other than the bandwidth to download them. I actually use The Petal Hack here in another game.
Another Fantasy game I'm fond of is Astonishing Swordsmen and Sorcerers of Hyperborea. Think of this as Old D&D without all the nasty hobbitses. It's reasonably cheap as a PDF, and also plays quickly (across a table, anyway).
I'm intrigued by Fate Core and Fate Accelerated Edition, but have never actually had a chance to play beyond the game development phase, so take my mention of these with a Small Siberian Salt Mine(pat.pending) in your hip pocket.
We are alive in a post-golden age of RP gaming - there are systems in unending profusion out there, including some indie games that are quite good. Lots of great suggestions here by others are also worth a look.
Whether you're runnin' a game or playin', here's hoping the dice are reasonably kind to you. Hit Community Chat if you want to hang out or have more general questions to ask.
As No. 2 might say, "Be seeing you."