Nami is a good support, she provides heals, CC, blessing, and her passive is the best!!
Nami's heal is the queen of the best trades (imo) as if you land it on your ADC while the enemy adc/support is close, it'll heal your ADC, deal damage to your enemy, and heal you back.
Nami's Aqua Prison, is the best, as it's a Stun unlike any other support champions, meaning, if you land it on your enemy adc, it really can save your adc or help your adc drastically.
Nami's Passive is like the best fitting mainly because it gives your allies movement speed which helps out so much early game as adc's tend to have about 335 Movement speed (which is not a lot) and its so useful against those annoying Thresh/blitzcrank hooks.
Nami's Blessing is great for an ADC or uses skill shots or has a strong harass. It gives them movement speed, and bonus magic damage on hit for the next 3 attacks. The blessing works incredibly well with ANY ADC. (Tip: If there's a Draven on my team, I don't tell them, but I max out this first lol.)
Nami's Tidal Wave, best ultimate. It can be used to ambush, or run away, or catch enemies, it even provides movement speed to allies that come into contact with it! It also comes along with a slow and a knock-up! Working well with Yasuo or Malphite!
And also, any allies that are touched by any of Nami's spell, get the extra movement speed.