Best probably depends on what you define as "best".
My best = cheap (to buy and run)
My best = works (alerts when it needs to, doesn't when it doesn't)
Do you just want to track for theft? Or anything else?
I've been using one of these 22mm / 40mm expanding Rib drain bung
) for 3 months on my caravan. No-one has nicked the van so it must work ;-)
Works well sends text as soon as van moves (if set to do so), can be asked at any time where it is by texting where to it, can send text if exceeds a certain speed. My van lives in a zero signal area so once you park it up you need to activate it with a text saying move. That doesn't work as no signal and if its moved 3 months later that doesn't work. May try reducing the speed detection down so it triggers earlier... its always active.