Well, there's the obvious synergy between Dervish and the monk. Load up on Vigorous Spirit, Live Vicariously, and Mystic Vigor and get healed for over 50 health every time you swing. Put like 3 points in earth prayers, and you get 50% evade and maxed health regen on top of that. If you only have healing prayers at 9, you can still get Mysticism to 15 and Scythe to 12 (or the other way around) with one superior rune. However, Mystic Vigor alone accounts for half of that healing, and with Mystic Regen and Vital Boon (with an enchantment saccing attack - I like Twin Moon Sweep), you can heal just as well without ever leaving the Dervish profession. Healing is a major component of the Dervish skills, so I don't think Monk as a secondary is that good of a choice.
Finally, the Paragon does have some good synergy as well. You're generating tons of adrenaline with that scythe that's not getting used, so why not use it on "Go for the Eyes!" to hit that huge max damage more often. Also, look at some of the finales in Motivation to heal you, give you energy, and remove a condition every time that shout ends (every 2-4 swings). However, Chants and Echos are not enchantments, so they don't go well with the mysticism bonus, and while you will be helping your team as well, there are better secondaries for increasing your own damage.
Mesmer is an option, for Illusionary Weaponry. However, IW cuts out that average 30ish base damage of your scythe, while allowing you to hit through things like blindness and weakness. The Dervish is weak against conditions, having only Avatar of Melandru (with it's 2 minute downtime) and Featherfoot Grace (only cuts their duration in half) to deal with them. But with either a Paragon or a Monk on your team, you shouldn't really have to worry about conditions anyway. Your job is to use that lawn mower of a scythe to wreak havoc on your opponent.
That brings me to what I consider to be the best secondary for a Dervish: the Elementalist.
The Earth Magic line has great synergy with the enchantment nature of the Dervish. Consider Kinetic Armor (renew an enchantment every 8 seconds to keep it up) for a huge armor boost, Stoneflesh Aura for unconditional damage negation, Sliver Armor with Extend Enchantments, or Armor of Earth in PvE. You can also pick up whirlwind/aftershock for some good damage.
On the other hand, you can pick up a Fiery scythe, and use the Fire Magic line for an incredibly offensive dervish. Consider Conjure Flame, Burning Speed with Featherfoot Grace to keep the foes around you nice and crispy, Mark of Rodgort in PvE where it might not get stripped so fast, then take Inferno, Flame Burst, or Star Burst [E] for extra PBAoE damage. If you have energy problems, use Raging Renewal or Vow of Essence [E].
All in all, I'd say the Elementalist has the most versatility as a secondary, while the Paragon has the greater potential for helping teammates in addition to yourself. I honestly don't see anything in the Monk secondary that the Dervish can't already do pretty well.