This build is characterized by low survival (though it applies to enemy as well!) and high mobility.
Everything contained in this build is based on my personal knowledge. I don't like to talk a lot so it will be a short guide.
*Note: Survi is fine if you're not noob.


----@NOTE : I WILL IMPROVE MY GUIDE TODAY---
06.02.2011

*9x Greater Mark of Desolation - better damage at the beginning, works fine with Youmuu's Blade.

*9x Greater Seal of Clarity - absolutely must-have runes. Allow us to harass enemies/spam parrrley/raise morale at creeps.

*9x Greater Glyph of Focus - cdr, very helpful. Mostly to reduce W/E CD's.

*3x Greater Quintessences of Malice - a slightly buff. We're not able to harass without them.
3x armor pen quints (9 armor pen)? Sounds great but this is just not enough, we rly need some crit chance.

9/0/21 (almost basic caster build)

+Buff Duration
+Mana Regen
+CDR
+Better survi

-Less damage

*Mastery points explanation:
+Archain Knowledge - makes our ulti still useful. 1 point in sunder = waste. It's not worthy.
-Greed - useless, to low amount of gold to pick that one.
+Haste, Blink of an Eye, Presence of the master - Improves our survi.
-Lethality - we have to devote something. It's slightly buff anyway, not a big deal.
-Havoc - u won't see the difference anyway.

*Chalice of Harmony (m res, eliminates problems with mana. you don't have to improve meki pendant if there are few ap chars in enemy team).
*Mercury Treads (m res, vs CC)

-Optional boots:
/Ninja Tabi (armor + dodge) / Lucidity Boots (30% cdr with those)

*Youmuu's Blade - it's simply good and cheap (it's an optional item, same as upgrading meki to chalice).

*Sword of Occult - "cheaper bf". Use your ultimate to gain stacks.

*Infinity Edge - must-have item, core.

*Phantom Dancer - everything we want.

After getting this sell Chalice & Youmuu's and purchase more DPS/SURVI items.

/DPS options:

*Phantom Dancer (second one)
*Blood Thirster (dmg + life steal, grants us more survi)
*Trinity Force (ias+dmg+hp)
*Frozen Mallet (hp+dmg)
*Last Whisper (if enemies are stacking armor)

/SURVI options:
*Warmog's Armor (lot of hp)
*Banshee's Veil (m reg + hp + bubble)
*Atma's Impaler (armor + dmg + crit c)
*Guardian Angel (2 lives, cat-like)

Note: I recommend: Occult + 2x Phantom Dancer + Infinity Edge + Blood Thirster + Mercury Treads.

Don't expect anything new over here, it's best possibility. Parrrley is your priority.
Last hit minions with parrrley to gain more gold than others or focus your enemies. Remember to spam raise morale at creeps after reaching lvl 2. Try to deny melee creeps or catapults.

Flash + Ghost - the only right choice.

Other possibilities:
Exhaust - maybe, but i prefer f+g
Ignite - NO, you already have ignite (GP's passive)
Clarity - with meki pendant? NO.
Heal - Lol, wut u just said?/
Teleport/Rally/Revive others - ******ed idea.

*PROS:
+great carry (no one can beat him in 1v1 fight)
+counter to vlad/mundo/swain etc
+insane damage (parrrley is like using smite at heroes, l0l)
+global ultimate
+haste + low buff for team (Raise Morale)

*CONS:
-he's squishy, like most of dps
-requires lot of cash

I hope you have enjoyed my guide to Gangplank, cheers.