If you cant get the [Plague Keewar] take whatever has the highest base damage.

This build is what I use to get consistent 200k AoE damage explosions out of Exodia Contagion (and over a million with occasional red crits). It is built for (melee) [Volt] with lots of power strength and active speed buff to get the most out of it but if you only want to use it for the occasional projectile instead of as your main weapon, attack speed matters much less. In combination with my melee [Volt] build its actual attack speed is 1.41.

If you dont have a Riven you may want to use attack speed for convenience, especially if youre using a different frame, or if you want maximum damage take Smite. With [Volt]'s speed buff you can get out a projectile about every second, without it every 2 seconds. Just tap jump twice, as fast as possible, to get minimum jump height and maximum fire rate. Swap around Elementals as needed depending on who you go up against.

I used to run this with [Primed Reach] since melee is usually my primary weapon but swapped it out for damage to get more out of the projectile which will have a lot more dps than just hitting stuff if you stay at the minimum range for Exodia Contagion to give you its 4x multiplier.

Since the projectile hits dont count towards your combo I went for maximum base damage, if you feel really spicy you can also use [Spoiled Strike], but at that attack speed you will have a very bad time if you want to use it to beat up enemies in melee as well. When used in conjuction with Energizing Dash as [Volt] you should be fine though, as it will let you keep the speed buff up 99% of the time, even if you happen to take a Magnetic Proc.