These days, these are the most common collars on the market.
DC40, DC50, T5 and TT15
But pay attention to the choice of equipment at the time of purchase as not all collars work with all Radios.
Astro 320 works with DC20, DC30, DC40 and DC50 and T5
Astro 430 works with T5 and TT15 (DC50 works after firmware update)
Alpha 100 works with T5 and TT15 (DC50 can be tracked if shared by a 430 star)