First and foremost, the valve's job is to pass air through into the tyre. For tubeless tyre fitting it needs to let as large a volume of air in as quickly as possible to pop the tyre up onto the rim with a track pump, Airshot style inflator or compressor. Once the tyre is in place, then the valve needs to resist getting clogged up with sealant (whose entire job is to clog holes up...). Therefore, look for valves with multiple egress ports/channels - side exits also allow the valves to be used with tyre inserts if so required. A large locknut is also desirable as these can often be difficult to undo after use. I particularly liked the nuts that had flat sides so that a spanner could be used. Some of the valves on test cleverly include a tool or two, such as spoke key, valve core remover and even a T25 Torx head, but I think that these are firmly in the 'nice to have' rather than 'necessary category - most riders will have these tools with them if they think they might need them and a valve cap is too easily lost to be relied on.