No single best amplifier here. What decides it is what you have to drive, what you need to plug in, and how much of the system you want in one box.
| Area | Better | Why |
|---|
| Power | A | Onkyo TX-RZ30 has more power - 100W against 80W into 8 ohms. That is about 1dB, which is a small step in loudness but real headroom on peaks. |
| Analogue inputs | A | Onkyo TX-RZ30 takes more analogue sources - 6 against 3. |
| Room to grow | A | Only Onkyo TX-RZ30 offers preamp outputs - which decides whether you can add a subwoofer or a power amplifier later without replacing the box. |
| Shelf space | depends | Denon AVR-X1800H takes up about 16% less shelf area (1471cm² against 1753cm²), and it is 63mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | B | Denon AVR-X1800H costs less - $849 against $1,100 at list. |
A Onkyo TX-RZ30 B Denon AVR-X1800H
depends neither is better in the abstract - it turns on your room, your system or how you will use them. Only specs published for both amplifiers are judged; anything missing on either side is left out rather than guessed.
Which should you choose?
Onkyo TX-RZ30 leads across most of the measured areas for a fairly modest premium, so it is the easier pick if the budget stretches. Denon AVR-X1800H is the one to take if it does not.
What a spec sheet cannot show: how either sounds with your speakers, how the controls feel day to day, and what the app is like to live with - which is where most all-in-one amplifiers are actually won or lost. Power figures are the manufacturers' own and are not measured to a common standard, so treat small differences as noise. If you can hear both, that will tell you more than any row above.