Grass types are super effective against ground and water and electric types are super effective against water and flying. Both types are even on strong points. The main factor would be which Pokemon would cover your team best. Luxray would compliment your team the best.
Luxray is weak to ground which is taken care of by Floatzel
Luxray will help your team against water and flying types since your team doesn't really cover these 2 types, except for possible ice moves on Floatzel for flying, or a grass move on Togekiss, which both aren't dependable methods.
Giving Luxray crunch will also help with psychics which seems like another potential weakness your team has.
Now moving onto moves for your team, the Pokemon you have are all very sweeping potential Pokemon so I'd recommend giving them all straight attacking movesets
Infernape@Life Orb
Ability: Iron Fist
Nature: Adamant
EV's 252 Atk 252 Spd 4 Def
Mach Punch: Great priority and works with iron fist
Fire punch: STAB, great physical fire move for Infernape
Thunder punch: A cover up move against water and flying types
Swords Dance: Good first move to get this guys incredible attack higher
Floatzel@ Choice Band
Ability: Water Veil
Nature: Jolly
EV's 252 Atk 252 SDef 4 Spd
Waterfall: STAB, chance of flinching, good physical attack
Ice Punch: Covers his weakness, good ice type move
Brick Break: Removes screens, good coverage for other types
Bulk up: Makes this guy a physical tank and great sweeper
Togekiss@ King's rock
Ability: Super Luck
Nature: Modest
EV's 252 SpAtk 152 Def 104 SDef
Air Slash: Main go-to move. Great chance of flinching with king's rock and super luck, STAB
Aura sphere: Never misses, great coverage against steel and rock types
Nasty Plot: Get that SpAtk up real high
Water Pulse/ Shadow Ball/ Roost/ Thunderwave: Your choice, coverage, health, or status
Lucario@ Life Orb
Ability: Inner Focus
Nature: Modest
EV's 252 Atk 252 Spd 4 HP
Aura sphere: Great STAB move, never misses
Psychic: Good coverage against fighting
Dragon Pulse: Coverage against dragons
Nasty Plot: To get special attack up
Garchomp@ Life Orb
Ability: Rough Skin
Nature: Adamant
EV's 252 Atk 252 Spd 4 Def
Earthquake: STAB, high accuracy, crazy powerful move
Stone Edge: Great power, covers ice weakness
Dragon Rush: STAB, can cause flinching, covers dragon weakness
Hone Claws: Set up and boosts accuracy which helps dragon rush and stone edge
Luxray@ Life Orb
Ability: Intimidate
Nature: Adamant
EV's 252 Atk 252 Spd 4 HP
Thunder Fang/Wild Charge: STAB, pick between power and recoil or weaker power but no recoil
Ice Fang: Covers ground weakness
Crunch: Good coverage against ghosts and psychics
Thunderwave/Light screen: Cripple opponent or assist the team