There are some filters that you can only get as Wratten gels. The #44 and #44A cyan filters leap to mind. They are just the ticket for achieving an orthochromatic look from panchromatic films. I have a couple of these with me whenever I'm working in the field.
Of the tri-color separation filters, #25 red, #58 green and # 47 blue, the last two are difficult to find in glass. I use both the #58 and #47 when needed in the darkroom for split-grade printing. I can also get almost a full grade more contrast from VC papers with the #47 blue filter than I can with full magenta filtration from my color head, so I use that when really needed.
I'm sure others have Wratten gels they find indispensable as well; hopefully they will add to the thread.
Best,
Doremus