The ones I like the best are the ones in the TLD7855, which are basically shock doctor foam (none reactive) with a plastic sleeve over them which feel a bit like the Sam Hill Knee Pads. Both of these have been crashed extensively and never had any issues. They're also both light weight, fit well, don't vary too much in comfort and performance with heat (the Sam Hill's can be a bit "edgy" when it's really cold when you first put them on).
The ones I dislike the most are the ones on the Racer Motion - these use the super thin 3DO - which is great weight wise, but about as comforting in a fall as sticking your arm in a condom and are utterly pointless other than for slides and scuffs.
Bought some Leatt 3DF 6.0 pads earlier this year to run solo and with other setups. Really nicely made and have a plastic outer, which is a must with anything "reactive foam" as reactive foam is shit in the real world for sharp objects which are what your elbows hit when you need pads, unless it has the external plastic to spread the impact.
Sized down on these due to all the report of them becoming loose. Not sure how much of that depends on user abuse of techniques to put them on and off, but mine did ease off slightly from being too tight as new. For actual riding, if I'm plodding I like them. If I'm going for it, I hate them. It's like someone's attached someone else really fat arms to my body and I can feel them blobbing about on my arms when you hit the gnarly stuff.