There is no single perfect scythe for *any* build out there. True of a normal PvE/PvP build or a farming build.
There are three mods you can have - a prefix, a suffix, and a damage modifier. You scythe will be named: "<Prefix> scythe of <suffix>" and your damage mod is called an "inscription".
For the prefix - you have three main choices, I normally try and carry three.
First is a vampiric - it has a -1 health degeneration but steals 5 point of health per target hit (steal means that it adds the damage to you target and adds that amount to your health). This is, by far, the best damage prefix in the game as life stealing bypasses *everything* and while your health regen is up an extra -1 isn't even noticed and normally *more* than negated by the +5 health/target/swing of your scythe (in this case, that should be 15 health per swing). Just note that you *can* die from the -1 health degen so swap out once you are done fighting.
Second would be a zealous mod - exact same thing as above except it is with energy instead of health. If you are hitting two or more enemies you are breaking even or gaining energy with this. This one is OK to carry around always, though you may need to swap it to something else if you need to regen energy.
And lastly you have "other" - namely nothing, sundering, or an elemental. Forget sundering unless you just happen to find it and have nothing better to put in there - vampiric will give you MUCH more damage. An elemental mod will do more damage against some critters - namely ones like warriors who are strong against physical damage (though elemental mods do less against critters like rangers who have extra armor against elemental damage).
As for the inscription - +15% while enchanted is by far the best. First, you need that extra damage way more than you need the extra 5 energy. Secondly as a 130hp dervish you are *always* enchanted so you will *always* have the +15% damage. You will not always be above 50% health - the +15% damage while greater than 50% health is more of the best general mod, though since a Dervish is reliant on enchants I would still almost almost always use +15% while enchanted.
And lastly, we have the suffix. For a 130hp dervish the only one you want is the +20% to enchant mod - you live by your enchants and they longer they are up the better. You can somewhat live with a lesser +percentage if you have too (say, the +20% is beyond your funds and you have not found one), but you should really strive for all your 130hp scythes to have one.
For me I carry three, all of them +15% while enchanted. A Vampiric Scythe of Enchanting, a Zealous Scythe of Enchanting, and an Icy Scythe of Enchanting. For the most part I use the last one because I'm too lazy to switch out, however there are runs where I need to switch or an occasional enemy where I have too.
