Oooh, several good'uns...
Bear in mind these are my personal opinion and it is only based on the add-ons I've actually got. I have no knowledge of add-ons that are out there that may be better, if I do not have them.
Some of these are freeware, some payware;
CLS A340-500/600 (payware)
AFS-Design Typhoon (payware)
Aeroplane Heaven Spitfire (payware)
Alphasim Raptor (payware)
Iris F-35B (payware)
The much talked-about Cessna 150 - freeware
Socata TBM700 - freeware (see featured downloads)
The recently-ish-released Beechcraft Kingair 350 - also freeware
Plus a few that I simply like for their quirkyness...
Hama's Queen Mary 2
Hama's Freedom of the seas
The 1960's series Thunderbirds 1 & 2 originally for FS2002... but they're fine in '04...
Hama's Hovercraft
Hama's hydrofoil ship
Hama's RMS titanic
Hama's bus