The Tamron 18-200 is my top choice here. It isn't perfect optically, but it is good, and it's such a good price and convenient focal range that I think it should be the lens you choose. The Sigma is very similar, but I prefer the Tamron. The Canon 24-105 is a FANTASTIC choice if you plan to ever get a full frame camera since it will work on both. But it's too expensive unless you buy used, and doesn't get as wide as the 18-200. Amazon has lots of used lenses available.
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