I haven't a chance to try out the Dervish, or to play Guild Wars for that matter..
But my favorite would have to be Dwayna, her avatar is just so beautiful!
im voting balthazar or melandru. 40armor and 33% faster run for like 60+ seconds is just hawt. Also Melandru's +200life and cannot be effected by conditions just stops alot of builds.
1) Grenth - Used ALOT on flagstand Dervishes to Remove the Protting that pressured targets usually get. This is obviously the favorite since Aegis chain and Prot Spirit are such popular ways of mitigating pressure.
2) Melandru - Condition immunity makes this one of the choices for a Ganking Dervish (Its harder to get you out or kill you), and its used at the stand by some teams in conjunction with Wearying Strike - though most of the ones I watched swithed to Grenth.
Balthazar's is ok for Running (or maybe farming undead?). Dwayna's is an awesome PvE one, and Lyssa is good on a situational basis (when you know you'll meet the +dmg conditon alot).
For PvE I usually stick with Balthazar. The extra armor and speed is very helpful when you have low AL and the enemies run like headless chickens. Melandru is nice too... but you need to be careful - depending on your armor/weapon setup, if you die once you won't have enough energy to cast it unless you have +energy elsewhere.
I'd say Balthazar of Dwayna for PvE. PvP is a different ballgame altogether.
I like dwayna for the looks and the healing bonus, i don't use it on myself but it's great to help keep melonni alive while saving my healer's energy a bit.
Balthazar looks like a good one but i haven't tried it yet. Melandru also would have good potential in areas where there are alot of conditions being applied.
lyssa and grenth seem like great effects, they just don't fit my game style as well at the moment. I think it's important to try them out and see what fits with your style the most
2 of the ones i use more often are Melandrus and Balthazar.
They are all good in certain places or against certain players/monsters/builds.
-Balthazar the +40 armor and 33% faster running for +60 secons is cool, especiall if you add a skill that makes you attack faster.
-Dwayna if its gonna be hex heavy this one is cool, the healing is a bonus.
-Grenth good to use against boon prots, and any opponent that uses lots of enchants
-Lyssa +10 nice! +dmg vs opponents activating skills, also nice
-Melandrus the +200 hp is always nice , good against condition teams/foes
I was gonna put a list on how i think which is better then the other one in most cases, but i coudnt, theres just way too many things to consider.