The sixth item is often down to what your opponents build / the champions they play, sometimes however it can be down to what your team builds / plays".

Frozen Mallet offers great chase potential on top of much needed health.

You should build this if you are against high mobility champions (such as Nidalee, Vayne and Singed).

Thornmail can really be a thorn in the enemies side if they choose particular champions

This item is extremely tanky against AD champions as gives power house reflective damage, amazing against auto attackers this item is best purchased if you stick on the enemy ADC (this forcing them to attack you).

Frozen Heart is a solid purchase for any mana using tank / bruiser

The best time to buy this AD Destroyer is when the enemy AD characters choose to avoid attacking you OR if they rely on attack speed (like Vayne, Jax and Kayle).

Banshee's Veil shield allows for good spell blocking when used against he right champions

This item works very well if you have enough damage and armor but need a little more magic resist, against champions like Teemo this item can completely win you the fight.

Maw of Malmortius is a great bruiser item and good tank item

The lifeline passive gives you huge potential of increased effective health against magic damage burst champions (like LeBlanc and Annie)

Locket of the Iron Solari works wonders against AP orientated teams or area of effect melting.

This item should always find a place in your team, if someone doesn't have it then they are wasting 230 damage shield ONTOP of 20 MR and 75% base regen... for the ENTIRE TEAM, by itself it has more value then any other item in the right situation, should even be taken against a team of just AD for the regen and shield.

Guardian Angel makes all the enemies fear your presence, to deal with you they need to risk focusing all their power on you for a few moments at which time your team will have made a dent in their power (and then you revive!)

Take this if you are ahead and your team is primarily AP, this is to give you the appropriate power-play opportunities and good time to get in the middle of their team and force their hand onto attacking you.

Last Whisper gives a spike of damage, it's a must buy for any ad champion that needs to kill bruisers / tanks.

Get this item if your team doesn't do much damage or if the enemy team doesn't do much damage, additionally if the enemy has decided to stack armor to counter you then buy this

Phantom Dancer is a scary item to deal with at some moments, it provides major damage through attack speed and crit chance, additionally it gives good movement speed and ignoring unit collision.

Get this item to take objectives and dispose of health stacking champions, works well against champions like Nunu that reduce attack speed (since you can get attacks in with this) Honorable mention to Statikk Shiv for potential in burst-sustained fights which are rare yet can happen.

Sunfire Aegis gives immense damage in an area whilst giving good defensive stats, since the area damage is magic it works exceptionally well on people of the opposite damage type (makes counter building harder)

If the enemy team stacks tight often and relies on summoned creatures / structures to win in fights then this can help melt and dispose of those threats, additionally Sunfire Cape gives solid defensive stats that allow you to survive to make use of this area damage.