Silhouette, size, then color when choosing a pattern.
Presentation, location, and stealth are much more important than pattern in the park.
Confidence is on both park and tailwaters. If you are not fishing with confidence you are probably not focusing as well on the presentation and location of your casts. We all probably have way to many bugs in our boxes, which means we are often thinking of what fly to tie on next and not how and where we should be fishing. Or at least I have found that to be the case on both watery venues.
I can not count the number of times I have stumbled up a park stream trying pattern after pattern only to realize:
1. If I would slow down I will quit stumbling
2. If I would slow down and fish the fly I had on where it need to be and how it needed to be there............I would catch more fish. Pattern is not always that important.
Tailwaters can be a whole new ball game. Floated the SOHO last fall. Possibly the prettiest day on the water. Sulphers where coming off everywhere and fish where actively rising. I tried dries, emergers, nymphs, and then repeated the process in differing sizes many times with no luck. Switched to streamers and had a great last part of the day, but talked to a guy at the ramp who said he caught his fish on bwo's....I didn't see a bwo all day. Got to focused on those little yellow easy to see bugs missed several hundred fish that I left rising along that river. Pattern can be very important.