@lukestokes.mhth (the witness account of @lukestokes) missed the least number of blocks among the top witnesses.
Witnessing Steem's block production happens to be the primary responsibility of a witness.
Other than witnessing blocks, Luke adds a lot of value to the community by educating us about cryptocurrencies frequently. His passion to promote Steem whereever he goes (and he speaks at many conferences) is priceless.
@timcliff is one of the most actively engaging witness who not only see technical side of the blockchain but also promotes Steem in beneficial ways.
I can say a lot about some of the people mention but Luke and Tim deserve more than what is written.
We need witnesses of different abilities and domains to have a balaced progress. Promoting Steem is important too. Only dev activity cannot take Steem forward.