Sorry to sound harsh about your existing lenses or at least two of them anyway
The 70-300mm will suffer the most especially at the tele end and still surprised they bundle the 17-85mm as the kit lens. Wasn’t great with the 50D and 15mp, so will fare worse at 18mp as some 7D owners have already found
Still think you need a standard type kit lens, always nice to have something versatile. The new Canon 15-85mm IS seems to get good reviews and gives a good range, also consider the Sigma 17-50 OS as well
Good lenses do make a big difference and I stand by my recommendation of the Canon 17-55mm IS, I wasted money buying others before i finally bought one and found it a marked improvement.
Admittedly I do not own any primes apart from a 400mm for wildlife, but between the 17-55mm and a 10-22mm I haven’t found myself wanting for anything else.