Micro Four-Thirds is the format for you if you want an interchangeable lens camera but aren't to keen on lugging around large and heavy gear. Benefits of MFT include a great lineup of small, inexpensive lenses that are brand-agnostic. Panasonic and OM System (formerly Olympus) both make great MFT cameras and lenses that you can mix and match as you please.
Since the format makes for smaller gear, MFT is a great system for crowd photography, as well as general, everyday shooting and they are great travel cameras as well, as they are so easy to bring along.
In terms of image quality, they are typically positioned between the smaller compacts cameras and the larger APS-C cameras.
Top Pick: Panasonic Lumix G9 II
Panasonic's latest MFT camera G9 II has pretty much all you can ask for from a professional cropped-sensor ILC. It has a sharp 25-megapixel sensor, a large viewfinder, great in-body image stabilisation (IBIS), flippy screen, dual card slots and a weather-sealed body. It also shoots video in 6K (60 fps), 4K UHD (120 fps) and 1080p at 300 fps. It's also the first MFT camera with phase-detect auto-focus, which is a big step up over the old contrast-based AF system for continuous tracking of a subject.
For a Micro Four-Thirds camera, the G9 II is not particularly "micro", though. It's quite a beefy camera, weighing in at a hefty 658g (23.2 oz). You will still see the benefit of Micro Four-Thirds with the many small lenses available, though.
The G9 Mk II is also...not cheap, so for most people, the original Lumix G9 probably makes more sense.
The original G9 has a 20-megapixel sensor, also with IBIS. Like the Mk II, it also has a fully articulating screen, dual card slots, and a weather-sealed body. Where you'll see most of the improvements with the Mk II are its better AF system and higher video specs. The original G9 tops out at 4K UHD, with slow-motion video in 1080p at 180 fps. The G9 and G9 II both weigh the same.
There is also a slightly smaller and lighter model called G90 (Europe) or G95 (America), that can also be worth considering. It is very similar to the original G9 but has only one memory card slot and a smaller battery, fewer AF points and some other downgrades. I does have onboard flash, though, unlike the original G9.
Panasonic Lumix G9
Older Model
Panasonic Lumix G90-series
Slightly Smaller Than the G9
Lighter alternative: Panasonic Lumix G100
The G100 (also known as G110) came out in 2021 and is the smallest interchangeable lens camera Panasonic currently offers. It was strangely marketed as a vlogging camera but is actually better suited for photography, with its large viewfinder that is completely redundant to anyone filming themself.
The biggest letdown with the G100 is its lack of in-body image stabilisation (IBIS), which is a big deal for videographers shooting handheld, but also affects stills photography, especially in low-light situations. For comparison, the older Lumix GX9 had IBIS, the same 20-megapixel resolution and was also smaller and arguably a better option, depending on your needs. At least, the G100 is 96 grams lighter, at 352 g (12.4 oz), and has a fully articulating screen, which the older GX9 didn't have.
Even smaller and cheaper options
As you've probably realised by now, to find more compact Micro Four-Thirds cameras from Panasonic, you have to look at older models. The reason for that is that the company now seems to mostly focus on the high-end segment and doesn't have any truly compact, pocketable MFT cameras with interchangeable lenses in their current lineup.
On my gear page, you can read about my two MFT cameras, the Panasonic Lumix GX9 and GX800, which are two small rangefinder-style cameras that came out in 2018 and 2017. Both those cameras are smaller than anything Panasonic currently offers. Unfortunately, they are no longer in production, but you can probably find them used fairly cheaply.
The GX800 is the smallest of the two but lacks IBIS and an EVF (which the slightly larger GX9 has). If you want something as small as the GX800 but with a viewfinder, take a look at the even older Panasonic Lumix GM5. Apart from an EVF, that model also has a hot shoe, but no built-in flash. The GM5 is from 2015, though, so don't expect any 4K recording; this one tops out at 1080p at 60 fps. The screen is also fixed with no flippage.
Best Olympus: OM System OM-5
Since OM System acquired Olympus, they have shoved the model names around for a bit, and the OM-5 is the direct successor to the Olympus OM-D E-M5 III. The old naming convention was a mouthful for sure, so OM-5 is certainly easier to remember and communicate. However, one could make the argument that renaming the Olympus brand "OM System" and then calling its models "OM with a number" wasn't the most inspiring or imaginative. To the regular John, "OM System OM-5" could just as well be the name of some industrial machine for manufacturing nuts and bolts. But I digress...
With the OM-5, you get a stabilised (IBIS) 20-megapixel sensor, a viewfinder and a fully articulating screen. It can shoot in bursts of 10 fps (mechanical), or 30 fps with the electronic shutter. The body is weather-sealed and is fairly light at 414 grams (15 oz). For video, it can record in 4K (DCI) at 30 fps, and 1080p at 120 fps, with no time cap.
A cool feature of the OM-5 is its so-called "live ND filters". These are built-in darkening filters that let you shoot at long exposure without having to carry physical filters with you. Perhaps nothing you'd benefit from for shooting concerts, but for other types of photography, like waterfalls etc, it's a very welcome feature.
There is also an OM-1, which is even more specced out but is in the price range of full-frame cameras, so for a smaller Micro Four Thirds camera, the OM-5 sits at a price point that makes more sense to most people.
Cheaper Alternative: Olympus E-M10 Mark IV
One of the last cameras released under the Olympus name was the OM-D E-M10 Mark IV, which came out in late 2020.
Visually, it is strikingly similar to the OM-5 but has many similarities under the hood as well. Like the OM-5, the E-M10 also has a stabilised 20-megapixel sensor. It has a viewfinder as well, but the screen only tilts. The burst mode is also a little slower, letting you shoot continuously at up to 8.7 fps with the mechanical shutter or 15.0 fps with the electronic ditto. You also lose the weather sealing of the more premium OM-5, unfortunately.
The video specs are also more basic. Here we're talking 4K (UHD) at 30 fps and 1080p at 60 fps. The recordings are also capped at 30 minutes.
There are some benefits to choosing a slightly less powerful camera, though. The E-M10 IV is slightly lighter, at 383 grams (13.5 oz), and has a tad better battery life than the otherwise superior OM-5.
Most notable is the massive price difference. The E-M10 IV often retails for about half the price of an OM-5, and it might just be the better deal for a lot of buyers.
Smaller Option: Olympus PEN E-P7
Olympus PEN series is their line of smaller, rangefinder-style cameras. The "E-P" PEN model names make it easy to call them EpiPens by mistake. Or maybe that's just me. Kind of glad they don't have a model called PEN I5, at least. Again, I digress...
The PEN E-P7 came out ten months after the E-M10 IV and has the same specs (20 MP, IBIS, tilting screen etc), albeit in a smaller form factor.
Unfortunately, the E-P7 lacks a viewfinder, but a benefit it has over the E-M10 IV is eye-tracking auto-focus, helping you stay focused on people more easily, which can be handy for concert photography. The little PEN camera is also lighter than its bulkier brethren, weighing in at a mere 337 grams (11.9 oz).