" Best lens for a hockey arena for Rebel XT or 30D"
Best is subjective. But because you need to shoot indoor you will need a fast lens would not consider a lens slower then f2.8. Most fast Canon lenses are "L" lenses and are expensive. They are great lenses and you wrote you wanted a "great lens". You will also need some reach no matter where you shoot from. However the longer fast lenses are big heavy and prohibitively expensive. A zoom lens may help if you are close to the ice when the action is near however the zoom lenses a full stop slower then the 135mm f2. If the arena is dark you will need the highest ISO possible that would require the 30D or 20D for ISO 3200. I don't like using ISO 3200 have only captured a few ISO 3200 images that I thought were usable after using NeatImage noise reduction. There are also third party lenses you might want to consider. I would want a Canon "L" lens 100mm or longer. Here is a list of the faster Canon glass:
Canon EF 50mm F1.0 L $ 2,599.99 2.2 lbs
Canon EF 85mm F1.2 L $ 1,499.95 2.3 lbs
Canon EF 85mm F1.2 L II $ 2,099.95 2.25 lbs
Canon EF 24mm F1.4 L $ 1,119.95 1.21 lbs
Canon EF 35mm F1.4 L $ 1,119.95 1.28 lbs
Canon EF 50mm F1.4 $ 309.95
Canon EF 50mm F1.8 II $ 69.95
Canon EF 85mm F1.8 $ 339.95
Canon EF 200mm F1.8 L $ Used 2-4K 6.6 lbs
Canon EF 100mm F2 $ 389.95
Canon EF 135mm F2 L $ 899.95 1.64 lbs
Canon EF 200mm F2.8 L II $ 659.95 1.68 lbs
Canon EF 14mm F2.8 L $ 1,799.95 1.23 lbs
Canon EF 15mm F2.8 Fisheye$ 579.95
Canon EF 16-35mm F2.8 L $ 1,369.95 1.32 lbs
Canon EF 24-70mm F2.8 L $ 1,149.95 2.1 lbs
Canon EF 100mm F2.8 Macro$ 469.95
Canon EF 70-200mm F2.8 L $ 1,139.95 2.8 lbs
Canon EF 70-200mm F2.8 L IS $ 1,649.95 3.5 lbs
Canon EF 300mm F2.8 L IS $ 3,899.95 6.0 lbs
Canon EF 400mm F2.8 L IS $ 6,499.95 11.7 lbs
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JJMack