The cheapest version of the series of shaper decoders that cost 1, boost strength for 1 and break subroutines for 1 with no further restrictions. ZU.13 Key Master is very cheap to start using at 1 to install, but pays for it in slightly lower base strength.

ZU.13 Key Master: 1 install cost, 1 base strength. Bonus: 0 cost if you have at least 2.
Gordian Blade: 4 install cost, 2 base strength. Bonus: Keep boosted strength for the rest of the run.
Torch: 9 install cost, 4 base strength.

While specialized decoders exist that are more efficient in specific setups, either of these will get you through any code gate you encounter for a reasonable amount of credits without any extra support.

Synergies:
If you only intend to use ZU.13 as a backup to your primary breaker, the slightly higher costs per run do not matter much compared to how many you save on installation.

Examples of efficient breakers where you may want to include ZU.13 as a backup:
Cyber-Cypher: Can only be used on a single server
Eater: Breaks everything efficiently, but prohibits you from accessing cards.
Passport: Can only break central servers.
Alpha/Omega: Can only break the first or last piece of ICE

If you get to 2, you can have ZU.13 installed without spending any memory. This requires you to include -boosting cards if you want to save the memory.

Examples of cards that increase :
Identities that start with 1: You only need one -increasing card to make ZU13 cost 0.
Access to Globalsec: Cheap source of . Usually better to pick a card that also gives you another benefit.
Dyson Mem Chip: Gets you extra and at the same time.
Rabbit Hole: If you want a lot of . Fetches another Rabbit Hole when you install it.

Cards that are better when you have high :
Creeper: Sentry breaker. 0 cost if you have 2.
Underworld Contact: Gain 1 each turn if you have 2.

Cards that are better when you have a lot of unspent :
Data Folding: Gain 1 each turn if you have 2 or more unused .
Overmind: AI breaker. Can break an extra subroutine for each unused .
Sage: Breaks code gates and barriers. Gains strength for each unused . ZU.13 can work as a cheap to install backup decoder for code gates whose strength is too high for Sage to handle.

I have a feeling we're going to see more of ZU.13 Key Master following the release of Spike and the other breakers in its suite. As a cheap cloud breaker with a reasonable influence cost and a cheap install cost, it essentially gives +1 strength to all the other breakers, and saves you from needing to blow one of your single-use breakers on a cheap code gate like Quandary or Enigma.