I'll start with the worst game companies because I love being negative.
Titus - Has this company ever made a good game? They shoveled the N64 with s**t like Superman 64 and Carmageddon 64 then went on to try to ruin the Dreamcast with some horrible Kangaroo platformer (is it called Kao the Kangaroo?) Haven't heard from them in a while. Lets hope they never come back.
EA- Yeah I know they have a ton of awesome games in the EA line, but EA loves to milk everything to death, censor their games, and not pay their employees. And don't forget about them trying to get a monopoly on sports.
Eidos - They gave John Romero 30 million dollars to make that shovelware Daikatana while giving warren spector's Ion Storm Romero's tablescraps to make games 10 times better, than they let it die. Don't forget how they their key franchise, Tomb Raider, die a horrible death by making every game progressively worse. Thank God it had a revival recently.
Ok now that we got that out of the way, time for the best.
Nintendo - Mario Zelda Metroid, Super Smash Bros, Fire Emblem, Advance Wars, Star Fox, Wave Race, Mario Kart, Pikman, Donkey Kong, F-Zero .. tons of amazing games, all at the top of their level.
And thats just their games, lets look at thier hardware innovations D-Pad, Analog Joystick, side buttons, Z trigger, Game Boy, Rumble Pad, Analog Joysticks, motion sensing controllers and most importantly DK BONGOS!!!

Also Nintendo saved gaming from the 1983 Atari disaster. No company has done more for gaming than them and Shiggy is the best in the business no question. You can question some stupid decisions they made like censoring Mortal Kombat or keeping cartridges for N64, but overall, no one can touch Nintendo.
Looking Glass - Thief System Shock Ultima Underworld Terra Nova. They did more for 1st person shooters than any other company, including id software, and noone even knows who they are. RIP.
Ubisoft - A game company that generally doesn't get the respect it deserves. It has a lineup that most companies would kill for. Rayman, Beyond Good and Evil, Prince of Persia, Far Cry, GRAW, Splinter Cell, Rainbow Six, Brother in Arms, King Kong, XIII (Highly underrated). When you see Ubisoft on the cover of a game, you know you are getting quality. Their good game to bad game ratio is one of the highest in the business, if not the highest. Too bad they have been slipping with their Wii releases though.