My tip is that know what game mode the weapons have the best uses on.
-Ball Game: Boomerang is the best, grenades, fan, and ice cannon (if used properly) are solid too. I like to have one stun weapon and one kill weapon for a balanced ball game team, but it also depends on the map. you want to have more kill weapons if you cant score from the side of the net.
-Hill: Fan is the best on most maps since most hills are surrounded by ice blocks,which fans make people slide away and earn you more precious time quicker. It can also be used immediately after you knock down an opponent. Grenades are solid too for clearing the hill, Fireball and acid bubble are decent. Toads can be used to stall and zone, but it depends on the map how well you can do so.
-Muckle: Grenades, fireball, toad bombs and acid bubble are solid, switch depending on the map.
Personally, I love grenades however it is easy to blow up a teammate with them too. Use when enemies like to cluster together and you have distance between you and everyone else.
Same goes with toad bombs, use from a distance. Primarily for zoning and disrupting when you spam from the edge of the map - annoying but effective.
Acid bubble is great for 1v1's, because you can get a stun on them, stand close to them, and acid bubble and get the kill as the stand up again (or still laying down). Also can be used to zone and can be spiked.
Fireballs are stiff to aim, can burn teammates, but the flame ignites other people upon touch.
IMPORTANT NOTE: If you are on fire, you can touch the water and relieve yourself of the burn without dying and giving the team a point. I see many people not take advantage of this/ don't know about this.
-Capture the Horse: Grenades, Fire Ball, and weapons that have kill potential. stun weapons are typically less effective because killing grants you more time, more time means greater opportunity to capitalize scores on.
My personal favorite is the grenade, because it can be used offensively and defensively. Also it's projectile spiking ability and it's large radius of splash damage are great. Say a teammate is fighting an enemy, you spike a grenade and blow them up, they get +10, but you get +20 so it's overall still effective. Be sure to learn the distance of when to use it close range without blowing yourself up.