When I receive mail I always look for the smaller envelopes since they are most likely to be a personal letter rather than a bill.
But I find myself with a few hundred of the Euro business size bought in a set with Original Crowne Mill paper. These are nice envelopes with a lines and good texture but I always feel that my letter is wearing camo.
Usually Office Depot standard #10 security envelopes that get a trip through the inkjet for some personalization. Occasionally a hand-folded envelope printed on card stock.
Just as a followup the next few letters I send will be in envelopes made from brochures that I picked up on my last trip. Turns out that a tri-fold brochure with a few cuts folds up nicely into an envelope.