This is actually kind of difficult... I think it'd have to be a toss-up between synthie organ, actual church organ and harpsichord. I enjoy the drawn-out, overwhelmingly powerful sound that organs make a lot; it's like they command your presence and attention and fill your entire being. Synthie organ and church organ pair better with different instruments. Standalone, you can take pretty much anything, play it on a church organ and I'll like it better than before; a real church organ is pretty much an orchestra in and of itself, after all. As for the harpsichord, I like the spiny, dwindling feeling it gives off. It's probably really hard to work into songs though.