summus

Summus, summa, summum is the superlative of superus, "upper", with superior in between. The positive is barely used, so in practice the ladder you meet is superior, summus.

The trap is partitive. Like altus, medius, imus and reliquus, summus normally marks a part of the thing rather than picking one thing out of many: summus mons is not "the highest mountain" but "the top of the mountain", and summa aqua is the surface of the water. When you do want "the tallest mountain", Latin says mons altissimus. As a noun the feminine summa is the total, because Romans added their columns upwards and wrote the answer at the top, and that is where English sum, summit, summary and the phrase summum bonum come from.