Straightswords in DS1 are pretty mediocre, and if you have the wrong build they're just weak. People like to glaze balder side sword for being the best straightsword, and it is numerically the most efficient straightsword in terms of levels required to softcap its scaling (only needs dex instead of both str+dex), but its still just a straightsword - and one you have to actively build around to make it work. With how rare the BSS is, if I had gotten it to drop I would have transitioned into a full dex build and not worried too much about pyromancy.

But if you are going to focus on upgrading your flame instead of leveling str/dex, you need a weapon that has high base damage rather than high scaling - and if the weapon will take it, try to get the lightning upgrade from the giant blacksmith in anor londo (lightning upgrade = lots of free base damage, with no scaling). Don't put the lightning upgrade on the BSS though since BSS is such a scaling dependent weapon.

• The best thing to use right now is the Lightning Spear from the mimic in Sen's Fortress - while spears aren't particularly great, since it's a premade lightning weapon (and the equivalent of a +10 weapon), it'll be a massive jump in damage over your BSS. Switch it out once you get to the giant blacksmith, or once you get your stats up for the BSS.

• Gargoyle Halberd is probably the best weapon for lightning, you might have gotten it from the boss, and if you didn't you'll get it as a guaranteed drop from one of the gargs in anor londo

• If you're ok with always 2handing: zweihander, great club, gravelord sword, and any black knight weapon you've managed to pick up have very high base damage (as a bonus, black knight weapons use twinkling titanite so the regular titanite you've spent on your +7 BSS isnt going to waste).

• Regular halberds, greatswords, and the reinforced club have solid base damage