For sheer RPG magical power, I'd have to go with Rydia of Mist from FFIV...
Though Tina/Terra from FFVI was pretty awe-inspiring by being part Phantom Beast/Esper (aka Summon), throughout FFIV, Rydia has access to every (cool) branch of magic: White (which, when she grows up, she ditches for more advanced attacking magics), Black, and Summoning Magics. So she can not only call upon Meteo but also Bahamut and Odin! Not many badasses like that in the FF universe.
For overall power and for kicking sexism in the ass back in the 8-Bit days, I think Silvercry is right, Samus Aran from the Metroid series is the girl to go for...
When most people first played Samus Aran's first game, Metroid, for the NES, they knew they were dealing with a force to be reckoned with. She's a bounty hunter with a kickass spaceship and awesome freakin armor, so naturally, people thought they were playing the average male lead that reminded them of something like Boba Fett. She was decked out with lasers, an ice beam, a weird ball form equipped with bombs, a spinning "screw" attack, and missles up the wazoo. Then, during the final stages of the game, you're stripped of your armor and WHUHHHH?!? Samus is a girl! Just plain awesome. She hunts space pirates, people! SPACE PIRATES!!! Though the name Metroid is pretty cool, I always thought the best of the series was the one with her name in it, for the oldschool Gameboy: Metroid II: The Return of Samus.
For being not only kick ass and kicking sexism butt waaaaaay back in '92, I've gotta give props to my girl from China, Chun-Li...
She was in the first Street Fighter 2 and has pretty much been in every subsequent sequal & spin-off of Capcom's legendary fighting series. Sure she had a kind of cheap throw, but she stole our hearts anyways...especially since she was the only girl in the game...though plenty of people speculated about Balrog/Vega (the name thing for the last bosses besides Sagat is kinda messed up...Balrog's the one with the claw which was renamed Vega for NA release...which was the name of the final boss which was renamed M.Bison...which was the name of the boxer [get it? M. Bison?] who was renamed Balrog). Now, we can get into a debate about how Chun-Li was just a cheap, wannabe Mai Shiranui from SNK's Fatal Fury series & King of Fighters series, so, I'll give props to Mai too...
She too was one of the first females in a fighting game...she was definately the first to be...bouncy...and tell you about it, "Mai bouncy!" Both Mai & Chun-Li get major props for having two of the best costumes for cosplaying girls to wear...*drools*
That second image is actually digitized...but I couldn't find a good pic...Man, going through cosplay pics made me remember Morrigan & Cammy...But I'll stop now...