I consider Delta to be the finest entry in Irem's classic series, indeed I obviously rate it among the greatest shmups of all time. It was fated to be overlooked somewhat, being exclusive to the Playstation meant it never got the arcade exposure it's famous predecessors did, and finding a home on a machine whose primary purpose seemed to be the delivery of true 3D games meant that it got pretty much ignored in the west.
There's a wonderfully scrappy, chaotic feel to Delta, partly through the Playstation's slightly wonky visuals and partly because of the sheer amount of stuff going on. That doesn't stop it being a very attractive game though - the multitudes of effects and superbly animated enemies are a treat. There are great touches such as the music changing to a muffled version as you head underwater on level 2, or the POV changing around sometimes to give a dramatic entrance to a level or enemy.
Naturally it's tough as old boots, but it does a lot to ease the pain of the player. For example you can't crash into scenery, and restart points are thoughtfully placed in order for you to quickly build up a decent offense. There is also a small selection of ships to choose from (foreshadowing R-Type Final's bewildering array), each with their own twist on the Force / Charge Shot. A chargeable smart-bomb type weapon rounds out the new additions.
Unfortunately Irem pushed the brand further with the mediocre Final, when Delta would have been a truly magnificent send-off for the series.