There are many answers to this question: traditional Vedic transits (gochura) are all in relation to the natal Moon sign; in the West, this is not followed at all, and transits are looked at in relation to the rising sign, conjunctions/parallels/aspects between transiting planets and natal planetary positions, etc. It is hard to say "which is best"-depends upon the practitioner's leanings, knowledge, insights, etc.
Me? I use (mostly) the oldtime Hellenist transit concept (Dorotheus of Sidon, 50 AD) and look only at transit conjunctions and Parallels of declination; plus, a variation of Vedic ashtakavarga doctrine, referred to as Sarvashtaka, which I personally believe to be the most reliably accurate transit-evaluating method (at least of the ones I am aware of!)