I love G5 as a power, form, whatever, but everything lorewise surrounding it was terrible and so was its foreshadowing.
If Ss1 came out now, it would be a lame straight powerup with no spice about it, nothing unique by current standards. But back when it came out? It was the absolute best thing in the world.
I don't know what power I like more, I do like G5 a lot, but looking at them on their respective context and frame, SSJ just stomps.