Tropica for me. Never had an issue with quality. I've had 50 pots of crypts kept in its transit box for 2 weeks and they were all perfect upon planting. They are generally more expensive that puts some people off.
Aquafleur are excellent value and all the specimens I've had have been good quality. Their Anubias is particularly impressive in size, usually.
My LFS has the two brands together in a decent CO2, filtered and well-lit system, so folk in my area are quite lucky.
Mail-order can be hit and miss unless you stick to these two brands from UKAPS Sponsors. Most other mail-order suppliers rely on imports from the Far East so quality may not be so consistent. This said, I recall Fluid Sensor Online sending us some nice specimens to a UKAPS event last year.
The key to buying from anywhere is to ensure the plants are healthy. This is usually acheived by getting them soon after delivery from the nurseries to the retailer, or getting them grown on nicely in-store i.e. in hydroponic systems.