Daytona Road is difficult at first, but it gets easy. The only challenge on Daytona Road is finding the proper braking points for each car and staying off the gas in the curves. It's a good high speed track, but there's no variety, and once you've memorized your entry and exit speeds... no challenge.
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Eiger is a lovely place to practice throttle control and car control... But it's mostly low speed stuff with only one really difficult high speed corner. But getting a lap right on Eiger is oh-so-satisfying.
Eiger Reverse is diff-i-cult. This is a track that'll teach you a lot about uphill driving... how slopes affect the balance of oversteer and understeer... a very good way to hone your driving.
Suzuka, IMHO, is the perfect place to practice. It's a mix of slow, medium and high speed corners... Setting up for Suzuka is a great challenge, and forces you to think very hard about set-up balancing...