Nothing is strictly better than Ack, but some tools beat Ack in specific ways.
A mostly-compatible clone of Ack. Written in C, mostly by Geoff Greer. It uses tricks like pthreads, memory-mapped IO, Boyer-Moore-Horspool strstr(), and PCRE's JIT to improve performance. It also supports some extra features, such as obeying gitignore/hgignore/svn:ignore. Geoff has written many posts about improving Ag.
A vim plugin for Ack. You can use Ag with this plugin as well. Just open your
.vimrc and add
let g:ackprg = 'ag --nogroup --nocolor --column'.
This program builds an index of functions and variables. Searching is extremely fast, but limited to what's in the index. Good for very large codebases.
Fast. Built into grep.