I like crit damage mods for the attackers, and speed mods for support. A good healthy mix is nice so you have options. Do whichever your closer to first.
Some good health mods are always useful too. Just look for those with a speed secondary.
Be a good idea to research how mods work too. If you farm a green mod, the secondary already showing will upgrade once. Blue mod's secondaries will upgrade twice.
Try to avoid upgrading grey mods unless yo have to. Exception being a speed arrow. Get everyone on your squad a +30 speed arrow and you'll have a big step up on the competition.