My favorite button devices are Lutron Pico remotes, but they require a Caséta Pro Bridge (or higher-end system like RA2 Select, RadioRA 2, or the fancier ones). That's not a cheap entry cost, but the remotes themselves are uncharacteristically cheap by Lutron standards and might balance out if you need a lot of button devices -- though there are more (and cheaper) options now than when I started down this path. Lutron does have a well deserved rock-solid reliability reputation, though. I've never replaced a battery in a Pico, which they say can last for 10 years (I'm going on 7, the longest I've had any).
If you just want a single button, the Aeotec (formerly SmartThings) Zigbee button is also one of my favorites, though it got a bit more expensive since Aeotec took over things: https://aeotec.com/products/aeotec-button/. Authorized sellers are listed on that page. The Sonoff Zigbee buttons of various generations are similar functionality-wise, but not quite as sturdy (and I don't like that they're apparently never listening for commands, though for sensors that's rarely a problem -- just makes it hard to know when their batteries actually die or if you just haven't used them in a while).
I've also been a fan of the Ikea buttons I've tried, or at least their newer ones that actually pair like regular Zigbee devices (Styrbar, Rodret, Somrig, Symfonisk Remotes Gen 2, etc.). I like Hue buttons, too, but I've only used them as part of my Hue system and not as directly paired Zigbee devices to Hubitat. They can be integrated either way now, though I've never found a use for them this way personally.
For Z-Wave, the Zooz ZEN34 and Zooz ZEN37 are both multi-button (2 and 4) devices that are often on sale for competitive prices, though less so recently than in the past. Other companies, like Minoston, offer similar products from what must be the same OEM, though they may not have the support reputation Zooz does.
Had some of these options existed when I started, I might not have gone down the Lutron path, but I don't regret that I did, either. I have a mix of many of the above, depending on the location/needs and just because I like trying new things. 