Is there any important missable item in Final Fantasy X?

Here's a list of all permanently missable items in the game.

• a no encounter weapon, as an equipment drop from Geosgaeno.
• a magic counter buckler, from the hovercraft merchant in the Calm Lands.
• the 0-Slotted Longsword can't be bought after a certain point.
• Master Spheres are limited in quantity.
• the four Al Bhed Primers from 19 till 22 are (technically) missable.

Also, certain non-items are also permanantly missable.

• Having Wantz in Macalania Woods selling 4 slot equipment
• You can't recruit Durren for your Blitz team anymore after a certain point.

For more information, there is a FAQ on GameFAQs giving detailed information for all of these items

The only thing I can think of is the part of Tidus' ultimate weapon. You can still get it later but the entrance to where the chest is is guarded by the Dark Aeon which is quite difficult to beat.