There are so many on my SIII 109 that it's hard to know where to start, but amongst the best are:
200Tdi engine;
Roverdrive;
MoD 110 side jerrycan lockers;
Wright Off Road sound proofing;
Defender inertia reel seat belts (big safety improvement);
4 x Defender front seats (special mountings for second row with altered tub floor for leg room);
Improved lighting (halogen and relay headlight conversion, front spots, twin rear fog and twin reverse lights);
Smartscreen intermittent windscreen wiper system;
Triple fuel tanks (42 IGal capacity), and finally, when I can complete the project:
Defender axles (CV joints, discs all round, better steering lock, 3.54 diff ratio, 6" wider track)