I've seen the list of supported devices, but it doesn't have any advice on which devices are better or worse for use with hubitat. A lot of forum advice seems to suggest that some brands or models are finicky or disconnect a lot.
So - what sensors do people recommend if you're buying today? I'm particularly interested in thermometer/hygrometer, smart garage stuff, and motion sensors.
I'd recommend picking a protocol (z-wave or zigbee), and then design a robust mesh of repeaters before adding end devices - like sensors.
Most of my sensors are zigbee. I find that to be more robust for my needs.
Thanks. I guess the same question applies, though: there is a list of compatible Zigbee repeaters; do they all work equally well and I should just find the cheapest, or are some better than others?
No doubt. The best ones in my experience aren’t on that list. The TuYA USB zigbee repeater. Small, inconspicuous, and about $12-15 for one on AliExpress.
Great! Thanks for the tip. Do you have experience with the other Tuya sensors? I see they have cheap thermometer/hygrometer and motion detectors, but wasn't sure if they were any good...
Many of them are supported by community integrations, particularly by @kkossev.
The thing is - The lack of official support can be a deterrence. So I’m picky about which community developers I trust - and @kkossev is high on that list.
I'm a big fan of Tired Reality sensors. They use AA or AAA batteries and just work
Is it possible you mean ThirdReality?
Yes , that’s what he means.
In general, I'd recommend sticking to things Hubitat has tested to work with built-in drivers:
https://docs.hubitat.com/index.php?title=List_of_Compatible_Devices
For actual opinions, see here for recent community favorites, and perhaps look for something like this again towards the end of this year if they decide to make it an annual thing. 
You'll find threads/polls where users discuss for (and vote on) their favorite devices in different categories, like switches/dimmers, motion sensors, contact sensors, etc:
Hubitat doesn't often make recommendations for or against particular devices, at least not formally, which I assume might be to avoid getting on bad terms with any manufacturer. The above reflects users' opinions, but keep in mind that everyone's preferences are different.
Regarding the "disconnect" issue you mention, that sounds like a problem many users report with certain devices that are not officially supported, sometimes for that very reason (or others), like many Xiaomi/Aqara sensors. Zigbee bulbs mixed with non-bulb Zigbee devices are also sometimes a culprit, though those bulbs still are on the list and seem to work fine among themselves. See this document for more on that: How to Build a Solid Zigbee Mesh - Hubitat Documentation (note particularly the mention of bulbs towards the end).
In my experience all the Aeotec and SmartThings (now aeotec anyway) devices, both zwave and zigbee, work well with my hub - except the doorbell siren which I think the driver isn't 100% right for. But it works. None of them are dropping off.
I have a mix of both radio types and if you get a weak signal on one just get something it can hop through, but don't bother with dedicated zwave repeaters unless you can find one everyone says gets hopped through. Mine was ignored and many posts say the same.
For lighting I have a Hue system with Bridge and use the community Hue integration app. If you're going to add bulbs directly to HE then be careful to look for recommendations as some are meant to be zigbee repeaters and don't do it right or something.
as long as you keep to the good brands like innur, new osram and ensure they are zigbee 3.0 you (like I) should not have a issue with this. All mine work perfectly.
I also have old Philips hue and osram lights which were causing lots of issues but now they are on there own hubitat hub (separate from the zigbee 3.0 lamps) I havent had one issue or drop off.
I don't think Hubitat even uses ZigBee 3.0. So even the old stuff can be great especially discontinued products from Centralite (Comcast) and Iris (Lowe's). They have to be bought on eBay but great prices can be had for the rock solid devices.
Example, There is absolutely no reason anybody using ZigBee on Hubitat should even consider any other contact sensor on the market when you can get 10 of these for $45. They are so incredibly solid and are the only thing on my mesh to have never failed. They even have temp built in so throw one outside, one in the fridge, one in you basement chest freezer. They cover all your temp/contact needs. Get creative.
Wow! You have unbelievably low prices there!
That is indeed a very good deal! Unfortunately, the seller doesn’t ship to Canada either…
Try other sellers, or search for ZigBee lot, or ZigBee bulk. That's how I've found all my deals on those discontinued Lowe's and Comcast devices. Though I guess all of the good price listings could be the same vendor, for some reason I remember seeing someone selling thousands of them in a single listing a few months ago.
Does anybody know of some things that could be done with this Sage Doorbell Sensor. I'm not sure if it's a chime, a button, or what it is, but look at the price on 12 of them! If you understand what this is please explain, or if you have thoughts on how this could be rigged to do things other than doorbell.
I've repeated this on quite a few threads now.... but my take would be.... Options.... Multiple platform support.... Flexibility..... Start Small....
Sensors / devices with their own "eco-system" often provide more options and features compared to those that rely on other hubs..... but I live in the wilderness (outside of the U.S. 
) . So for me, motion sensors like Hue motion sensors and temperature / humidity sensors like SensorPush offer their own options outside of HE while still allowing integration with HE when not paired directly with HE (admittedly SensorPush requiring a driver I wrote...). The same is true for my Kasa plugs, while I can pair them via the Community driver, I can still access them via the Kasa app. My blinds are in the same category, I can control them via the Bond App.
Separate to cross-platform redundancy / control, start small. If you take on a recommendation from someone in this thread or elsewhere, pick a room or situation to test out your decision. If it works, great. If not, change tack and deck out another room with an alternative and compare. I'll be honest and say I haven't always followed my own advice... but it sounds good in principle....
Above all, enjoy the ride....
That is such good advice.
I'd suggest starting here regardless of what sensors you purchase...
https://docs.hubitat.com/index.php?title=How_to_Build_a_Solid_Zigbee_Mesh
https://docs.hubitat.com/index.php?title=How_to_Build_a_Solid_Z-Wave_Mesh
Download the Hubitat app
