I have experience with the following third-party OS-es:
1. Qmeg. Have been using this one for quite some time when I did not have a Turbo Freezer. It gives a (software) freezer and a very nice ML monitor. Also a few other cool things like High Speed Sio and a ramdisk driver which even lets you boot from ramdisk, which is actually quite handy when you are a coder. I have used it a lot, and I have very fond memories of using Qmeg. IIRC I was at version 4.04
2. MyBIOS. This is an excellent OS for the use with MyIDE and MyIDE ][ ... it reaches an incredibly high compatibility rate and it adds all kind of welcome extra's to the MyIDE interface (like hot swapping partitions, something I still miss on the IDE+) ... Only downside: there are quite some versions floating around and it is for a novice user sometimes hard to decide which is the right version for you. The MyIDE ][ interface is nice, but in combination with the MyBIOS OS it is a VERY powerful interface. Not tested, but I believe MyBIOS also supports Sio2BT (see also OS 3 in this list)
3. Sio2BT OS. This is actually a pretty stock Rev. 2 XL/XE OS, but with an adapted I/O handler for use with the pretty cool Sio2BlueTooth interface. When you have U1MB you won't need this, since FJC added excellent Sio2BT support to his already fabulous alternate U1MB BIOS. Sio2BT is a nice invention which gives your atari a wireless connection with a mobile phone, tablet, or computer with blue tooth. It can boot XEX games and ATR files. With this OS you have a hardware solution for the slightly different I/O timing that is needed for this type of communcation (with stock OS you only can boot through XBIOS, and that is not what you want if you do more than just booting XEX files)
4. Hiassoft high speed OS patch. This is quite a nice OS replacement too. In fact it is not a replacement, but a tool that can patch your Rev. 2 XL/XE os so you have a high speed SIO routine as soon as you powerup your atari computer. I guess because of copyright Hiassoft does not share the OS itself (perhaps it is available somewhere?) ... it is always nice to be as original as possible. So an OS close to rev. 2 XL/XE os is (IMHO) the best choice. That is why I use -again- the U1MB with the High Speed SIO built in the PBI driver FJC created (he should be rewarderd with a medal for that alternate U1MB BIOS, but that is a different story).
I hope this helped.