What class are you, for starters? Since you said 2H slashing after 20 I'm assuming you're either a Pally or an SK.
Assuming Pally, there's no decision; you want Sword of the Morning. Pair it with a Sarnak Battle Shield or a Clay Guardian Shield and you'll be rocking.
http://wiki.project1999.org/index.ph...of_the_Morning
If you're a Shadowknight, your best bet is probably Noctivagant Blade, which is basically the SK variant of SotM. It has a better ratio and stats, but no proc.
http://wiki.project1999.org/Noctivagant_Blade
For a Warrior, while they're not as good as other weapons early on due to the damage cap, you can't go wrong with two Wurmslayers.
http://wiki.project1999.org/index.php/Wurmslayer
There really aren't any cheap options for a Pally, but for a Warrior or SK (assuming they can use Piercing), you might try Obsidian Shards if you're on a budget.
http://wiki.project1999.org/Obsidian_Shard
Finally, if you're a Pally, you should save up for a Ghoulbane. You'll need it for the Fiery Avenger quest, and it'll make you an ABSOLUTE TERROR against undead when you are high enough to proc with it.
http://wiki.project1999.org/Ghoulbane
BTW, the damage cap is 20 for levels 1-9, and 28 for 10-19, although I've been able to hit a bit harder in the teens as a Pally thanks to Str buffs, as hard as 37.