my understanding was that you have the BTS goal if the most expensive formation on the table was destroyed.

this means that if the most expensive formation was not on the table for whatever reason*, it would not count, and BTS would drop to the next most expensive.

highlighting the dangers of big aircraft units, such as this example

Harbinger bomber is worth 400 points, it is the most expensive unit in the army, and thus, the BTS. the second most expensive unit (an infantry formation worth 375) is wiped out on turn 3. since, when victory conditions are calculated, the harbinger has disengaged, it is not counted, and the BTS becomes the infantry formation.

However, if the infantry formation was still alive, but the harbinger bomber had been shot down with AA fire, it would count as the BTS, because it was dead (so in effect, if your most expensive formation is an aircraft, you effectively have two BTSes)

if you're concerned about gameyness, having it be possible to deny the enemy the BTS entirely is the more gamey option, taking a popcorn swarm army, and keeping your 225pt BTS unit in reserve is not going to hamper the army particularly much by comparison. i'm fairly certain that the intent of the FAQ was to prevent that sort of shenanigans

(*this includes spacecraft who never really show up, aircraft who have disengaged, units that have yet to teleport in, units in reserve for any non-necron army, and units that have hopped inside an aircraft transport that has disengaged)

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~Every Tool Is A Weapon, If You Hold It Right~

Exactly so Jaggedtoothgrin; although an army list may may have other more expensive formations off-table, they do not count as far as evaluating goals is concerned, so they cannot be considered the BTS formation until they actually enter the table.

As you say, this FAQ is intended to prevent a player deliverately keeping the most valuable formation permanently off-table in order to deny his opponent the BTS goal. Indeed there has been extensive debate over the practice of deliberately holding back the most expensive formation until the 3rd turn in order to make it virtually impossible for the opponent to destroy it and gain the BTS goal!

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However, EpicBattleBaggz was asking which was the 'easiest' Objective to get, and the answer depends on so many factors that it is almost impossible to determine - the BTS may be easy to achieve in one game and almost impossible in the next; for example, a Great Gargant is one of the hardest formations in the game to kill, but if the Ork player leaves it in the open against an IG army containing three ShadowSwords, some Death Strikes and a bit of "Dave Thomas luck", it could be toast by the end of the 2nd turn.

The point is that it is this constant variation in the various balances that makes the game enjoyable.