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Narrow band and line filters work particularly well at large exit pupils, which generally means 2" eyepieces. My biggest suggestion is to get a 2" filter from the get go, it will save you money when you realise you need to upgrade from a 1.25" filter.
DGM, Lumicon, Astronomic, Baader, they all make good filters. The pass band on Astronomic filters is wider than the others, so contrast is a little less, but the extra light may be more appealing to some people with smaller scopes. Baader updated their OIII filter to 10nm pass band, and in performance is almost identical to the Lumicon OIII. The Lumicon is still the best OIII (my opinion), but as a value proposition the Baader OIII is hard to beat.
The Lumicon UHC and DGM NPB are the top narrow band filters going, and one of those should be on everyone's buy list. I owned both for quite a while and doing comparisons was a lot of fun. They are more similar than dis-similar so I eventually sold one off. You could buy either with confidence, knowing you have a great filter.
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