Depends what you need. The SEI s905x4 boxes are good general purpose boxes. They’re google/neflix certified, support AV1, all DV formats, and have strong CoreELEC support.
The s922x/z boxes have significantly faster SOCs, but are older than the s905x4 and don’t have AV1 support. The AM6+ & U22XJ are also not google/netflix certified, and use Android instead of AndroidTV.
The 2nd gen FireTV Cube has all the streaming app certification (Widevine L1, Playready, Netflix) same processor as the Am6+/U22XJ. Can be found for 1/3-1/2 the price, but has to be bootloader unlocked (possible on any new/sealed unit). It’s also limited to one USB2 port.
All the above boxes support Dolby Vision FEL if you care about that. Then there are the newer s928x boxes. The s928x has faster single core performance than the s922x/z, but slower multicore performance. They support AV1. There are a number of these boxes that support DV, but none play DV FEL. Also none of them are google certified and all use Android (not ATV).
check out this the recommendation thread.