For the game I'm currently running, I use a heavily modified Old School Essentials crossed with Dungeon Crawl Classics. As the players are (with the exception of my wife) all first-time D&D/RPG players, a simple "old school" ruleset had my preference and I have a (Dutch!) copy of the Mentzer Basic set from the 80's. I didn't want to burden the players with choice between dozens of feats and a myriad of character races and classes, so the old school way seemed the best way.
That said, I did file some of the more "tough to explain" serial numbers from the ruleset. We use ascending AC and attack bonuses, for example. But also, we dropped the vestigial 3-18 ability score, instead only using the modifier (each player received 5 points to distribute among their various attribute modifiers).
In addition, I have taken a liking to certain embellishments and baroque elements that Dungeon Crawl Classics has added to the game. For example, Magic-Users, Elves and Clerics in my game don't lose a spell after casting. Instead, they make a spellcasting check modified by their spellcasting ability, aiming to score 10+spell level to cast. A failure means the spell isn't cast that round and a 1 means the spell is lost for the day.
Furthermore, I like the Mighty Deeds mechanic for fighters and the corresponding Deed Die. Though Marcus the Fighter has yet to reach 4th level (and the raise in attack bonus), I have already laid out a system for adopting that mechanic. Instead of a static attack bonus increase, Fighters receive a greater bonus die to add to their attack at level 4 and every three levels thereafter. They can also, during their round in combat, state to perform a Mighty Deed such as backing the enemy into a corner, tripping their opponent or swinging from a chandelier. The Deed succeeds on a 3+ of the bonus die.
Last but not least, I uniformed the XP progression for all classes. XP in the world of Flintlock Fantasy is gained by spending 2d6 x 100 gp a week carousing. The amount in gold spent is the XP gained that week. On average, that means 700 XP per week. I laid out a chart to see how many weeks it would be desirable to adventure before reaching level 2 and each level thereafter up to level 14. I then calculated the XP gain needed for each.
There are various other minor changes. Flintlock weapons, for example, are the dominant form of offense and armor has had some flavor changes too. Those will get covered in an upcoming post.