Do you mean darker than those series? X is probably the darkest Showa tv series, although the monster designs in the 2nd half get goofy. The early Heisei series from Kuuga to Blade are all pretty serious and dark, and generally actually sustain that atmosphere throughout, with the humor being fairly down to earth. Among more recent series, Gaim and Build start quite goofy but actually have tons of plot movement throughout and end up with really dire serious scenarios later on. There's also Amazons, a shorter series aimed at adult fans (and homage to Amazon, but completely unrelated plot-wise) that has a constant dark and gritty atmosphere throughout, with almost no moments of levity.
I haven't watched enough Ultraman to really make recommendations there. I guess depending on what you mean by "serious/darker" even the OG Ultraman and UltraSeven could work. UltraSeven also leads to bunch of Seven focused spin-offs which were mostly made for older audiences (and are directly connected to it story-wise, so you need to watch the original unlike with Amazons).
