Foreword:
The information contained in this post is a combination of my experiences and what I have read on WoWhead. The only other thread about this title was a half-assed attempt by Rohi (no offense intended). All drops are about one out of three, but I've noticed a consistant pattern in which I won't get anything for a handful of kills, and then suddenly multiple drops in a row.
Be sure to pick up Repeatable-Quest-Helper, it helps more than you can imagine.
So, you want the Diplomat title do you? Great! Here is what I did to attain it. Please note, I am alliance, however this does not matter until the Kurenai grind, the horde equivalent is Mag'har.
Timbermaw
First, you'll want to grind Timbermaw rep. I would recommend starting ASAP, if you're already level 70/80 (or the mobs are grey) tough-luck, you'll have a longer grind. Always kill named furbolg, they give rep all the way to exalted.
Start with the quest Feathers for Nafien (level 55ish) at the northern end of Felwood (very tip of the map), directly west of the quest giver is a lake, grind the mobs there for feathers and The Deadwood Totem (you can't turn in the totem until neutral). After you have the totem, you can go turn in your feathers, or grind for more up to you.
When you're satisfied with feather farming, go on to winterspring (through the tunnel, do NOT kill any furbolg or you suffer rep loss! Also note, the tunnel goes to winterspring and moonglade, use your brain.) and accept Winterfall Activity (level 58ish) from Salfa. Once you've completed that, you can turn in Winterfall Spirit beads in groups of 5 for Beads for Salfa (little better than 1/3 drop rate). Keep grinding on these mobs till you get an Empty Firewater Flask and A Winterfall Ritual Totem.
It will take 700 beads to go from revered to exalted, so save them! Turn in your Empty Firewater Flask, The Deadwood Totem and Winterfall Ritual Totem quests after you hit revered. (it is entirely up to you though.)
From now until exalted, you will be grinding in loops here (note, black is initial run, purple is where you turn around and finish):

Yes, the purple line does go into the mountain, there is an easy path up.
Kurenai
For horde, do Mag'Har Quests and turn beads into them! (I have no experience with Mag'Har, sorry.)
This is the second Faction you'll want to hit up, as it also requires mob kills for rep. The easiest rep you'll ever make is by doing the Orebor Harborage and Telaar quests, which will net you about 4.5k and 10.5k rep respectively. However, these are not needed.
Once you hit neutral (grinding mobs or questing) you can pickup Fierce Enemies which leads to a repeatable version where you turn in Obsidian Warbeads 10 at a time. Mob kills will take you all the way with this faction, so turn in your beads as you please.
You'll be grinding on ogres at the Laughing skull ruins:

Sporeggar
Here comes the fun stuff!
To raise your rep level to friendly, you should be doing The Sporelings' Plight and Natural Enemies.

Accept the quests from the Accept marker (Fahssn), and until friendly do them/grind in the spawning glen (west of the marker). Once you hit friendly, go to sporeggar and accept the quests Fertile Spores (about a 1/3 drop rate) and Now that we're friends (both of which have a repeatable version.).
Once you have those two quests, make loops following the yellow line, grinding on Marshwalkers and Sporebats for fertile spores. Each "run" will take little under 30mins as a 69 feral specc'd druid, will net you about 3-4k rep. (usually I pick up about 10-20 fertile spores) Don't forget to do the Now that we're friends quest! Unlike the Fertile spores repeatable, you need to accept the Now that we're friends repeatable each time you turn in!
BE AWARE: This path skirts the horde town. There are threshalisks that like to play dead on the northern part of the route. The naga are a commonly farmed area, 80's have tried and failed to kill me several times.
Conclusion
This is what I, as a feral druid, used to attain my diplomat title, I cannot gaurentee that this is the fastest way to attain it, and your mileage may vary. It may be faster to just grind the naga quest for sporeggar rep, but I found it too boring to continue doing for longer periods of time, so I worked on that little path that grabs them and Fertile spores.
Enjoy your painfully attained title!
Suggestions!
Originally Posted by
Diabloss39
I suggest doing Sporeggar first instead of Kurenai if your doing this while leveling up, as its more in order. Also I got to exalted with Sporeggar mostly by running through Underbog and grabbing the Sanguine Hibiscus. But This is a nice guide, and I'll rep it.