In order of increasing cost:
1. Replace only the damaged crank arm, particularly if it's the left crank arm.
2. Buy a new identical crank since you can use the existing bottom bracket and no changes to the shifters or front derailleur will be needed.
3. Change to a new compact cranks with the required bottom bracket and, possibly a new front derailleur.
Shifter compatibility will depend on the model shifters you have. 9-speed 105 and Ultegra STIs were both double and triple compatible. 10-speed 105 is both double and triple compatible but Ultegra is specific to one or the other. However, a triple STI can be used adequately with a double crank by locking out the granny ring with the fd's inner limit screw.