what filter(s) to use while shooting against the sun (ie. sunsets) in order to avoid those reflections of light on glass elements?

"Clean" or filterless will give the best results. Putting a filter on will just add more reflections.

And make sure your lens is nice and clean - give it a good cleaning right before you shoot the shot.

It's pretty much impossible to avoid. Zoom lenses have SO many air-to-surface internal reflections given the number of elements inside. Fixed lenses have far fewer problems but they aren't immune either.

add a nd.6 or nd9 grad for one. Tiffen 4x4 filters are good. You will not be able to shoot a clear sunset - way to bright - but with clouds, smog, haze - you can. The auto iris tends to under expose 1 to 2 stops... and be very contrasty compared to my sony pd150. I have to goto manual and add a few stops and have custom profiles boosting shadows for any detail. This camera does not like sunsets like the old PD150. However, it can be done.

This is a sunset I shot this evening.

http://homepage.mac.com/justin.carlson/~~temp/sunset_timelapse.mov

I used two filters. A B&W screw-in circular polarizer and a B&W screw-in graduated nd.6 filter.
Iris: Auto
Shutter: SLS 4
WB: 5000
Gain: -3
Built-in ND: 2

When this clip starts out the iris was at about 9.5 and by the time the sun just disappeared it was at 1.9 and the sky naturally got dark after that. The clip goes for another 40 seconds as it gets dark... but I didn't export that.
Hope this helps.

Jason,
you still have that violet dot in lower right corner;that's wat we are taking about how to eliminate it;

As far as I know, you can't eliminate it. It's just a by-product of using filters. The best you can do is find a area that will minimize that lens flare the most.

Sunsets are very tricky... if you center the sun - you will have less flares. Overall, the ex1 is very good at flare control vs the SonyPD150. if you can tilt or angle the filters in some mattboxes, you can sometimes remove the flare if it is from the filter. Otherwise, in post the trick is to do a picture in picture of a clean spot of sky nearby (or black in the ground) and move and overlay the pic over the flare - add some feather blur to the 20x20 pixel box and you are set. This will not work with a moving camera, but with a non moving camera, it can work as long as nothing is moving through the flare - birds, clouds etc...