er: sweeter, sunnier, longer, younger, hotter, frostier.
est: the sweetest, the sunniest, the longest, the youngest, the hottest, the frostiest.
more: more comfortable, more interesting, more favourite, more wonderful.
most: the most comfortable, the most interesting, the most favourite, the most wonderful.
er/more: preasanter/more pleasant, narrower/more narrow.
est/most: the preasantest/the most pleasant, the narrowest/the most narrow.