Take Hapur bypass starting from Delhi >Hapur > Moradabad bypass > Rampur > Lalkuan > Nainital. This road is fine except some pot holes after Moradabad till Rampur. For about 20 km you gave to go slo (Read More)w and watch the road very carefully, rest of the road condition is awesome. Another alternate route is Delhi > Hapur > Moradabad > Thakurdwara > Kaladhungi > Nainital. This is longer by about 30-35 km, but the road is smooth. Never take the Tanda > Kaladhungi road, as there is no road for a stretch of 25km.
This would mostly depend on your date of arrival and departure from Nainital, and where you are staying. Please refer to the Places to Visit Tab above for details on tourist places in Nainital. This (Read More)section also has a map that will show you the locations of these places, which should help you plan your trip better.