The "best" varieties of clematis are those you prefer :-) Pruning group 3 or the late flowering hybrids tend to great for beginners and also have less issues with the possibility of clematis wilt. 'Jackmanii' is one of the oldest hybrids and a long time favorite of many clematis growers. Usually starts blooming in early summer and continues pretty much until frost.

There is also a dedicated Clematis forum here with many long time clem growers as participants and I'm sure you will get lots of other suggestions if you were to post there as well.