Is the MSX the greatest for 8bit equivalent RPG games?

Yes and you can get great deals on the PC-8801 from Yahoo JP. Sub $300 for a full setup. I seriously considered this and the x68000 before settling on the MSX. One of the big reasons was the MSX community is so outstanding.

I've been also interested somewhat of the PC-88 computers, but looking closer at them, it seems that they're not so good as MSX with choppier gameplay and awful looking scanlines on the graphics. So, no point really trying to get hold of them playing the same games with worse presentation (and lacking translations and community). PC-98 is of course a different thing and worth getting, but I'm not myself much interested about 16bit computers or consoles.

So, what might that be? Some games that MSX didn't have? And my mind is not gonna be changed on that subject - I have seen enough of both, and I have pledged my loyalties already!

Valis 2 is clearly better on MSX2; more colours 8 vs. 16 (and better used on MSX), scrolling, performance and the music / sound effects are all better also. Emerald Dragon seems to be somewhat faster on PC88, Dios I haven't really seen on either machines. But the PC88 being better on few games is a rare anomaly. If we asked the question other way around: which games would be better on MSX, the list would be very long.

The vast majority of the PC88 games use the YM2203 soundchip, which has 3FM+3PSG sound channels, mono.

It's a nice FM chip with its 4 operator FM channels, but OTOH only they support sine waveforms and the polyphony is pale when compared with the 9 channels of the OPLL. The OPLL btw, supports two types of waveforms.

Those are two different beasts, and thinking that one or another is better is up to one's taste. I particularly prefer the OPL soundchips used on MSX because the sound is smother, has more accords and harmonics. The OPN series always sound a bit harsh on my ears. But that's me, YMMV.