I think it's all the aspects combined that make a connection really powerful, like a good recipe.  

But if youre asking about intensity, look to Pluto and the 8th house.  

I think Chiron is overlooked too often as well, Chiron with Venus or Jupiter or Moon can feel like a fairy tale.

As Elsa said Mercurial connections can be good for communication and relationships need communication in order to stay healthy. Never underestimate Mercury in synastry....if you have strong Mercurial connections, especially double whammys, you can communicate without words...yes, telepathy.  And whats more special than that?

I also think parallels from one persons personal planet to another persons planets feel just as strong as conjunctions. When you are obsessed or someone is obsessed with you, and you can't find any major Pluto aspects in a regular synastry chart, might  want to check the declinations and see if Pluto is parallel Venus, Moon, or Mars...

Interesting.  There's a man who's obsessed with me. I notice his Mars, Saturn and Pluto, all in Scorpio fall into my 8th house.  My Pluto squares his Venus.  His Venus is conjunct my Moon.  We have Moon square Pluto double whammy. 

Must be the Pluto and 8th house stuff that causes the intensity.  I try to avoid him.

I admire you for being able to be friends with a man you are attracted to.  I would find it too uncomfortable.

Thanks. But I'm trying to distance myself from him. It's becoming too much for me.

I'm not sure if it's the Pluto/Moon/Venus/Mars synastry or the Sun/Venus synastry that's overwhelming.

Thank you, Iriishka. I will pay more attention to house placements from now on and look at individual charts. The examples you gave really help a lot.

Will look at what I activate in their chart and they activate in mine.

This really helps, iriishka.