In an ideal world, version 6.x is better than 5.x, version 7.x is better than 6.x, and so on so forth. However, we're not in an ideal world.
Cisco builds different "trains" in parallel. Currently, the active trains for CUCM are 5.x, 6.x and 7.x.
This "multiple trains" approach is a compromise between market demands and compatibility. In order to support new features, big changes need to be made to the infrastructure (e.g. database schema). Sometimes, the changes are so big that it's impossible to be compatible with previous versions. So they introduce a totally different "train" to lower the risk.
It's really hard to tell which train is the "best". Of course, newer train would have more features. But they also have more requirements. For example, CUCM 6.x is compatible with CUPS 6.x and 7.x. But CUCM 7.x is only compatible with CUPS 7.x.
On each train, there are many "sub-versions". For example, on 6.x train, you have 6.1.1, 6.1.2 and 6.1.3. Read the release note carefully. Some versions won't be able to upgrade to another train. For example, CUCM 6.1.3 won't be able to upgrade to 7.0.x (because of different database schema)
On each sub-version, there are also "build-numbers". e.g. 126.96.36.1990, 188.8.131.520, etc. Build-number is the most confusing part.
Generally speaking, build numbers should increase in 1000, such as 184.108.40.2060, 220.127.116.110, etc.
CUCM is built on Linux OS. Whenever Cisco release an OS security patch, they'll increase the build number by 1000. This is called PSIRT patch.
Remember CUCM is an application running on Linux. OS patch does not contain any CUCM bug fixes. Any bug fixes would be in ES (Engineering Special). ES versions would be indentified by the last three digits in build numbers (e.g. 18.104.22.1682)
OS team and CUCM (application) team are two different teams. When the OS team release OS patches, they don't include any application patches at all. But the version number was increased by 1000.
Quiz: 22.214.171.1240 and 126.96.36.1992, which one is "better"?
Answer: it depends on how you define "better". But most of the people would think "less buggy" is better. When they say "less buggy", they mean "less bugs in CUCM". If that's the case, 188.8.131.522 is better. Because it has ES number of 112, which means it fixed quite a lot bugs. While 184.108.40.2060 has no CUCM bug fixes at all (it contains OS patches though).
Confusing enough? I don't know which genius invented this version schema. But that's the way it is. If you try to "upgrade" 220.127.116.112 to 18.104.22.1680, it'll fail with some vague error messages. You have to open a TAC case to understand why it failed.
Interesting? Yeah, that's the way to keep TAC engineers' jobs. :)