The lowest performance series is the Atom series and the Celeron series.
Core i3 / Core i5 / Core i7.
Core i9 will be launched in 2018 for desktop CPUs.
There’s also the Power m-Core Core m series for ultra-book laptops and tablets. Performance is roughly around the Core i3 series. It focuses on saving battery life and reducing heat dissipation. (The Pentium series does not refer to older models like the previous Pentium III / Pentium IV / Pentium D, but to Pentiums launched in 2006.)
(There are other CPU series for special use, only the regular series are selected.)