Welcome to SPCR!!!!
Im going to suggest you read SCPR Fan Roundup #6: Scythe, Noiseblocker, Antec, Nexus, Thermalright
, in that review they compare some of the best 120s on the market today,
1) Noiseblockers M12-S1
Really good fan with a top end of 850 (not 750 like NB rates it), imo this has the best tonality of all 120s i have ever tested, but its expensive. If you need more cooling look into M12-S2 also reviewed on the above.
2) Scythe Gentle Typhoons AP12
I own AP15 and AP11, and both of the them undervolt very well, you can drop them around 300rpm, really good fans, middle price compared to the noiseblockers, but still not cheap. The problem is they are already discontinued by Scythe, so getting a hold of them is hard. Ap13 might be a good bet since they are rated 1150rpms, if they are like my AP15/11, they should also undervolt well.
3) Nexus Basic 120
This is a pretty solid fan in the review was beaten by the two above, still a very good fan for what it costs, and it undervolts well. Check the review for more info.
4) Scythe Slipstream or glidestream
Wonderful cheap fans, their tone is very good, the only downside of the fans is that they are low mftb, so they wont last as long as the above. There are mulitple RPM versions, from 500rpms to 1900rpms, so chose depending on how much cooling you need and how much noise you will be willing to accept, personally i would suggest not to go above 1200rpms as it will be loud, but will depend on how much you are overclocking and how much cooling you do need.
If you are in a budget and want to spend the least you can, i would go with Nexus Basic 120 or Scythe Glidestream. If you can increase you budget some, Scythe Gentle Typhoons would be my pick, and if you want to spend heavy on fans, i would go with Noiseblocker M12 S1/2.