go-to

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Being a player on an athletic team who is relied upon to make important plays, especially in clutch situations.
• adjective Being the person or thing most trusted to accomplish a purpose.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Desirable
• adjective Reliable; likely to perform in difficult circumstances.