Over rated -
SWIV - Even now people talk highly of it, but it's just such a dull repetitive shooter with rubbish weapons, unlike the Amiga version of Silkworm which is fantastic.
Frontier - I just don't get Elite or Frontier, about the only classic that I don't understand the appeal of. It's like flying from spreadsheet to spreadsheet.
Under rated, maybe Banshee, it's not as often mentioned when people talk about the best shooters on the system, and to be fair some of the bosses are a bit rubbish, but I just love the levels and the character of the game.