R2-D2

Battle Droid (Trade Federation Army): Rubbish at fighting; great to cut down with lightsabers. Defeated Jar-Jar Binks and the Gungans in the Battle of Naboo (Starwars.com)
Droideka (Trade Federation Army): Shields and twin blasters make them dangerous even for Jedi. (Starwars.com)
Advanced Battle Droid (Trade Federation Army): Like Battle Droids on steroids. Still fun to cut down though (Starwars.com)
Quarterstaff Droids (Separatist Army): Wield sonic quarterstaffs well enough to take on Jedi. Make for great bodyguards (Starwars.com)
General Grievous (Separatist): Is he a droid? is he an organism? Is he force sensitive? The only thing we can say for sure is that he's dangerous (Starwars.com)
Imperial Probe Droid (The Empire): Looks scary, but not as scary as what inevitably follows (Starwars.com)
Medical Droid (The Rebellion): Great at replacing arms that have been cut off in lightsaber combat. Put everyone at ease by giving them a sympathetic look (Starwars.com)
Hoover Droid (The Empire): Follow Storm Troopers around looking busy. May secretly be the main enemy. (Starwars.com)
K-2SO — Imperial security droid (The Rebellion): Strong, coordinated, intelligent and because of reprogramming, sans-filter! Almost human (Starwars.com)
C-3PO — Protocol Droid (The Rebellion): Annoying but occasionally useful. Such as when the teddy-bear-like Ewoks installed him as their deity. Proficient in over 6 billion forms of communication (Starwars.com)
R2-D2 — Astromech Droid (The Rebellion): Pint-sized dynamo who keeps fighting the good fight. A solution for every fix and great at rescuing the day (Starwars,com)
R5-D4 — Astromech Droid (The Rebellion): Exploded unexpectedly moments after being introduced. Had he not, the galaxy would have been very different. (Starwars.com)
BB8 — Astromech Droid (The Rebellion): Great at scooting around and even better for merchandising. BB8 is like a baby R2
Hollywood
Droids uncooperative? Really?
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