2.53.0 Release 2 is the "current stable" release.
2.53.1 is being worked on, and is in beta phase.
2.55 has many new additions, but is in alpha phase. However, it's built from 2.53, so basically everything that's stable in that version is also stable in 2.55, and it's generally only the new features that might have some flux or odd behavior to them, so don't be put off by the "alpha" if its features are appealing.
The latest builds are always available at https://www.zeldacla....com/downloads/
2.50.2 (and lower) are OFFICIALLY UNSUPPORTED, so no one should really be using them anymore (aside from non-windows-users, as the new versions only have Windows builds thus far). This means you should update at MINIMUM to 2.53.0, though 2.53.1 is basically built right from the same code as 2.53.0 with a few new things WIP; so it should be just about as stable. 2.55 then is in alpha, but alpha does NOT mean 'unstable', it means 'new stuff is still being added'. We hope to move 2.55 to beta soon, at which point new stuff (aside from anything we put in the spec for beta) will NOT be added, and the majority of updates during beta will be bugfixes for the features. As Lut said, mostly anything that is stable in 2.53 is stable in 2.55; the only stuff that has a real chance to be unstable are new features.
All that said, it is ALWAYS recommeneded that WHENEVER you change ZC versions, always back up your .qsts for any quests you are developing.
For PLAYING quests, if you notice any oddities that are different between versions in the same .qst, please file a bug report in the proper subforum.