Entirely depends on how much you're willing to spend.
I'm going to assume that you don't want to spend much since you have a T2i so I'd recommend the Canon 80-200mm. You can buy one used for around $70. It's obviously no L lens but it gets the job done.
For a beginner like yourself, I'd recommend taking the route of buying a cheap lens mainly because it'd suck to drop a lot of money on camera equipment only to lose interest in photography a year later (especially if you're on a budget).
I actually picked up that lens off of someone on here for $25 last Fall. I didn't have the money to be dropping much on a lens and didn't have a long range at the time. Here I am almost a year later and haven't had the urge to buy a nicer lens (and I use the lens a lot - mainly for photos but some video as well). It worked great for filming skiing. When the video is compressed to 720p it's damn hard to tell the difference in quality vs. my 18-55 kit lens.
I'd say get a 24-105 to replace your stock lens and use as a medium tele. You could leave it on 24/7, and eventually add a wide angle or longer tele as needed. 38.4-168 (equivalent) at f4 with IS is pretty good,and you should be able to shoot most things with that. Keep the canon lens until you can afford a 24/28 equivalent if you need it (that length is good for traveling/indoors or anywhere you can't take a step back/group photos).
treadmills are the trick to everything. fat chicks, skiing on grassy flat back yards, OK GO videos.. - jibnut
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