In at least my dialect it's a flap like the R, but moved to the teeth. IPA transcription is [ɾ̪]. This only occurs intervocalically: VdV where d is a dental stop.

I am from the US. In most In many North American varieties of English we habitually pronounce the /d/ sound and the /t/ sound as a voiced tongue flap when the vowel before it is stressed and the following syllable is unstressed and either starts with a vowel or else the entire syllable is only one of the three consonant sounds [l], [m], or [r]. When we speak carefully we sometimes give the sounds /d/ or /t/ their full value instead of the flap pronunciation. A few of the many words that I usually pronounce using the flap pronunciation include: latter, ladder, letter, better, bettor, bedding, ladle, sidle, rider, writer, reading, bottle, subtle, bottom, girder, garter, barter, starter, larder, warder, saddle, addle, adder