Twitch is one of many assassin heroes that have been built into the game. This rascally rat has been retroactively ignored for far too long. It is time to feel the power of the long tailed torrent of torment! As for myself this is my first guide and hope your comments will help to improve the quality. As a summoner, I started playing Eve and then moved to Akali and Twitch. Since Akali has become very popular for being banned in ranked games, and because of the nerf to eve's stun, I find Twitch is now a viable alternative to those who are inclined to sneak around the map causing havoc.

I choose marks of insight for better damage from Expunge.
Seals of evasion and Gylphs of warding are for survavability purposes.
Quints of swiftness are to increase your mobility on the map.

I choose flash for the obvious escape mechanism. Flash can also be used offensively to bridge the gap on an otherwise unreachable opponent and then slow them.

The ignite skill is used to add even more damage to your already explosive burst potential.

Other possible spells choices are:

Cleanse: This is a good choice if the other team has massive cc (Note: you can also switch to Mercury Treads at the end of the game to help counter such a team to help combat these disables)

Ghost: This spell increases your movement speed by a great amount and is very useful for quickly traversing the map. A good choice if your wish to specialize in counter-jungling during the game.

Teleport: Sometimes throwing this into the mix is always good to plant wards and backdoor at critical moments in the game. Also if you are solo in your lane, returning as soon as possible using this summoner spell is a good way to prevent damage to your turrent as well as minimizing loss of experience and money. As well, this spell can be used to defend towers as well as teleport right to the action.

Spells not to get:
Revive: This is essentially a worthless skill. Unless you are a tank and your revival means the life and death of your team you should not take this ability. Even in this case there are spells that might be more helpful in preventing your death in the first place.

Exhaust: Yes this spell is a good counter to the other teams's carry, however due to the slow effect of your debilitating poison ability, this part of the spell is wasted.

Fortify: Again, your job is not to defend towers, although that is part of the game that you will have to do. Your job is to roam and gank and this skill will not help you in fulfilling your goal.

I go with 23/0/7 with points in archmage's savy and archaic knowledge for additional magic damage.

The reason for the items I choose is because ability power stacks 100% with every shot Twitch takes against his opponents. Since this is the case ability power is important to do the most damage possible from magic damage. Attack speed is another important factor for Twitch due to his ability Expunge. At six stacks (after six attacks) you can set this skill off to do additional 360 damage. This plus your ignite skill pack some serious nuke potential for ganks. The passive from malady helps to stack damage on even quicker and this this is the first damage item you should strive for due to each additional ability power getting the passive benefit from this item.

There are three items that have the natural combination of attack speed and ability power that are the keys to this build. Malady, Nashor's Tooth, and Guinsoo's Rageblade. Banshees veil can be completed after these to give you survivability late game, or sooner if you feel you are being focused on early. The last item of the build that synergizes best with the other items is Madred's Bloodrazor, due to its bonus magic damage.

Pros:
-great ganker and scout
-very mobile
-forces enemy to spend money on wards
-useful ultimate for team fights
-efficient counter-jungle

Cons:
-fairly squishy
-can be cc'd
-wards/oracle can shut you down

Early game, the ability to last hit in your lane is essential. Also, setting up ganks with your team and getting kills/***its will send you on your way to amassing the funds you will need to dominate. Remember timing a gank for the right moment is essential. Do not feel you have to wait out your stealth's entire duration just to get a slightly bigger attack speed bonus. If your opponents have pushed their lane and the time is right, start the ambush and get those kills!

Late game, you can easily clear each wave with just auto attacks or use your spray and pray to clear a creep wave in just a few shots due to its pass through effect. (I would only recommend doing this at level 16 due to the 75 second cooldown, otherwise I feel the waste of not having this skill available is not worth the quicker gold.

Twitch is a great scout and anti-jungler. As for scouting you can follow an enemy hero who goes out of his lane and he will likely lead you to the rest of his team. Also, feel free to ward the enemy jungle and annoy the hell out of the enemy jungler for some free buffs.

As well, buying an oracle early in the game just after your malady and roaming the river to kill enemy wards is a good way to shut down early map control and set up easy ganks for your jungler, as well as taking dragon every time its up.

For a synopsis of what this guide is about, the following applies.

Steath > Gank > Win

But seriously, Twitch is a great carry hero that can make or break the game depending on how you play him.

Have fun and play smart fellow summoners!