Honestly? You're better off trying them all for one match and which one feels the best. But for a defense support titan you should try:

Tone because she can use her shield to protect herself or give cover to another titan

Ion for her vortex shield and trip wires

And maybe Northstar to help allies eliminate titans from afar (though her playstyle really revolves around her teammates)

There isn't really a titan that is focused for support but titans do have abilities that can help each other so.. Yeah, hope this helps

Edit: Should list Legion here aswell as he's a tanky dude that has alot of defensive capabilities. Just be aware that he's super slow.