DavidTheWinMankySteam\steamapps\common\Team Fortress 2\tf\resource\tf_english.txt
"TF_Competitive_Rank_1" "Recruit"
"TF_Competitive_Rank_2" "Mercenary"
"TF_Competitive_Rank_3" "Field Mercenary"
"TF_Competitive_Rank_4" "Veteran Mercenary"
"TF_Competitive_Rank_5" "Infiltrator"
"TF_Competitive_Rank_6" "Operator"
"TF_Competitive_Rank_7" "Raider"
"TF_Competitive_Rank_8" "Guerrilla"
"TF_Competitive_Rank_9" "Agent"
"TF_Competitive_Rank_10" "Field Agent"
"TF_Competitive_Rank_11" "Master Agent"
"TF_Competitive_Rank_12" "Assassin"
"TF_Competitive_Rank_13" "Field Assassin"
"TF_Competitive_Rank_14" "Elite Assassin"
Well, at least top rank has elite in name.
I do hope they change some of them, if someone tells you they're FA they could be rank 10 Field Agent or rank 13 Field Assassin. If you tried adding the second letter you get FAs and... FAg.
As i said these are temporal names, and i would probably see people using the numbers 1-14 instead of the names.