I recommend Godox Flashes. If you plan to use the flash on-camera, you only need the flash itself. If you want to use the flash off-camera, you need two flashes or one flash + Trigger. You must buy the flash designed for your camera brand.
Godox XPro II Wireless Trigger
You will need a trigger to fire your flash off-camera.
XPro II for OM/Olympus & Panasonic
Budget
Godox TT600: If you’re just getting started on a tight budget this is the way to go.
Pros: Powerful, easy to use, built-in wireless transmitter and receiver. Works with all brands.
Cons: No TTL. Uses AA Batteries. You need to purchase a second flash or wireless trigger to use off-camera.
Godox TT600 for all brands: https://amzn.to/3Vzmav1
Compact
Godox TT350: While this is a smaller flash, it should provide enough power for almost any indoor situation and will do well outdoors except on the brightest days. It’s packed full of features you normally only find on more expensive systems. Get the one made specifically for your brand to take advantage of automatic TTL & HSS exposure. I would consider this an entry-level enthusiast flash that can be expanded into a full system of flashes should your needs grow. I recommend you get the transmitter/controller as well. This will allow you to use the flash off-camera. The flash runs about $85.
Pros: TTL, HSS, built-in Transmitter/controller and Receiver, compact size.
Cons: Not enough power for bright days, uses AA batteries, difficult menu operation, You need to purchase a second flash or wireless trigger to use off-camera.
Godox TT350 for Olympus/Panasonic
Enthusiast
This is a full-size flash with enough power to handle most situations, even in bright sunlight. Being full-size allows it to have a nice large LCD control panel. This will come in handy because the flash also has a built-in transmitter/controller and receiver built-in. It’s compatible with the Godox X system of flashes. The lithium battery makes for quick recycling times and will last all day. As of this writing, flash runs about $229.
Pros: Powerful, TTL, HSS, Transmitter/Controller and Receiver built-in, easy-to-use menu
Cons: Non-standard battery, but it has fast recycle times and lasts all day!