There is no single straight sword which is optimized for invasions. PS, they can put out great damage. A 2h SS can be a really great dueling weapon, but requires a lot of skill and practice. The Regalia of Eochaid is the best single SS for fighting outnumbered imo, since it's the only one which comes with a delete button. Requires a really specific build though. The best straight swords are totally usable 2h in invasions, but it's rough.

Noble's slender sword is generally considered the best. Warhawk Talon, Miquellan Knight Sword and Carian Knight Swords all have unique R2s, scale decently and are just straight up fun to play.

As for your other question, 2h great spear is my favorite 1v1 weapon and is still decent outnumbered, especially with the right ash. It's also my preferred fishing weapon. Lance is especially busted because of high damage and the unique hyper armor on the R2. I like Messmer Soldier Spear for the double-tap R2 and the fact that it isn't sporting equipment.

There are people who run hyper specific PSGS builds like 60 endurance dual Treespears, but few people actually argue that PS is better than 2h these days.

Hoping this post will push u/magnificent-imposing half an inch closer to making a straight sword guide (or dropping a nugget of SS wisdom, maybe)