I guess it depends on your interests and any physical limitations on your line. I love the London Tram and found it a compelling and absorbing model to build, but it is of course, much taller than any other stock, so you need to consider any bridges or tunnels on your line. The new Buenos Aires tram however is simply superb, with high levels of detail and plenty of white metal parts included.
Occre Buenos Aires Lacroze Tram Model Kit
If you want to motorise it, I've designed and shared on the 3D printing page some axles supports that fit the Slaters wheels and Apple Tree motor-gearboxes. These make it a simple screw in conversion, with plenty of room underneath for batteries and a speed controller - or indeed just fitting some pickups to the wheels.