It's hard to say that any of them are trouble-free. One flawed update from Roku can sour one's opinion of any of them. However, I always prefer the lager boxes over the sticks or tiny boxes. Packing electronics that produce as much heat as Rokus do into a tiny package is just a bad idea in my opinion. I've owned the majority of models and the only one that's given me trouble is almost certainly because of overheating.
I don't care for the sticks because RF interference due to its close proximity to the TV can be an issue and I don't like that you're beholden to Roku if you need a remote. I also don't care for Wi-Fi only models. The newer sticks and the Express 4k+ can use an Ethernet adapter, but it's an extra expense. Of course if there's no way to run an Ethernet cable to the location of the Roku then it's a moot point.
So, my vote is for the Ultra.
The last time I was in the market, I avoided the Express models because you see more problems posted here in the Community for the Express than any other model. I don't know whether this is because the Express really IS more problematical, or if it's that, as the lowest priced product in the line, there are just that many more of them in service.
Over the years, I've purchases many Roku devices, for self and family. I've had personally one die, under warranty, and Roku replaced it. I had a family member's die, but it was 7+ years old, and a first generation model (2010-2011 era).
Apart from that, I've had no issues with my Roku devices working. Then again, I've always purchased the higher end models. I did purchase some entry level devices for family members (all the grandchildren), but they worked well. But for myself, I've always purchased the top of the line (Roku XDS, Roku 2XS, Roku 3, Roku Ultra, whichever was top at the time).
I will say about the sticks that I've tried them in the past. The only one I really liked was the recently discontinued model, the Roku Streaming Stick+, model 3810. All the others were underpowered. I've not tried the new Streaming Stick 4K+ device, so I can't speak to it from personal experience.
DBDukes
Roku Community Streaming Expert
Note: I am not a Roku employee.
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