I agree wholeheartedly with Kreeft. US's elite program (Dividend Miles Preferred) is most generous with even it's lowest-level elites, and their highest level elites get unlimited domestic upgrades (subject to availability, of course).

US often runs promotions which give passengers double qualifying miles, as well as challenges whereby passengers can qualify for elite or next-level elite status by flying a certain number of miles within a 90-day period.