Tough to say, but I’m partial to the 70-200mm canons although you may be hard pressed to find a really high quality piece of glass at that range.
http://www.amazon.co.uk/CANON-70-200-4-0L-USM-Lens/dp/B000053HH5/
If I would recommend anything I would recommend building out your kit of lenses slowly over time. You can always check ebay or craigslist but you need to be extra careful there. I’ve only ever sold gear there but I’ve never bought there.
Don’t forget you can always get an extender if you need to increase the power of the lens.
Extenders are awesome for that:
http://www.amazon.co.uk/Canon-14XEF-II-1-4x-Extender/dp/B0000C4G56/
What I would also say is to not focus on your kit as much as you focus on the images themselves. Some of the greatest photos ever taken we’re created with old cameras that are probably worse than what we are shooting with now!
I hope that helps.