You know I love these types of questions because they are always based on opinion, gear, level, playstyle and mob type.
Are you looking for a single 20 sec fight?
A series of 20 sec fights with time (30 sec, 60 sec, 90 sec, etc.) in between combats?
A series of 20 sec back to back fights foe a short time (15 min, 30 min)?
Or A series of 20 sec fights, back to back, for a very long duration (3+hours for example)?
What type of "support" would this toon have? Will ther be a toon or 2 hat can give buffs to regen mana or end?
What type of gear will you be equiping on them - will it greatly increase vital stats, will you be farming rares or special items for them, or will they get a basic, generic set of gear -prestige, non-prestege?
Will you be maxing AA's or will they be running with the 250 free max, they 1000 silver max, or are they heroic...
If you are running Gold account, and have some way to regen end, Id say Berzerker would be your best bet.
If you are running a Non-Gold account, you may wish to look into a Wizard (plus they have ports).
If your mobs are "humanoid" maybe a max AA Ranger using headshot...seen many 3 sec fights with that...
Another thing to think about is what ELSE could your DPS toon do for you that you are lacking atm (i.e Wizard-ports, Beastlord-Paragon,pet,buffs,melee and caster, etc.)?
If you fights are already lasting only 20 sec, do you REALLY need that much more DPS?
I know that this probably doesn't help a lot, but it does factor... casters are very mana regen dependent, melee types are very gear dependent, and we all know how precious that endurance regen can be.