No single way of talking is enough on its own -- most couples who make this work end up leaning on all four channels, each for what it's actually good at.
Phone calls
The most immediate option, but time-limited and expensive per minute in a lot of cases. Best for the conversations that need real back-and-forth -- working through something, or just hearing each other's voice on a hard day.
CorrLinks (email)
Not instant -- your partner needs computer time to send and read -- but it lets you write at length without worrying about a clock running. Good for the longer, slower conversations that don't fit into a phone call. (See the pinned CorrLinks guide under Guides → Communication if you haven't set it up yet.)
Physical mail
Slower still, but it's the one thing that's tangible -- a photo, a letter they can keep and reread, isn't something a call gives you. A lot of couples treat mail as the place for the things that matter most, precisely because it takes real effort to send.
Visits (in-person and video)
The one channel with actual physical presence. If distance makes in-person visits rare, video visitation (available at most facilities, doesn't count against the monthly in-person minimum) can fill the gap between trips.
The pattern that tends to work
Quick daily-ish contact through calls or short messages, longer real conversation through email or letters, and visits as the anchor points you're both counting down to. No single channel carrying the whole relationship -- that's what burns people out.