the "bestest" place would be to make an enclosure right on the a-pillar above the defrost vent for both the tweet and mid...but...whos going to do that? lol
the only problem i have with the stock tweeter location is the reflections off the windshield since it creates a delay in the waves path to your ears. you can hear it at certain frequencies with the stock speakers.
practically speaking, the best place (if youre keeping the speakers in the doors) would depend on the tweeter itself. for example, you dont point metal domes at your ears...that hurts lol. those would be ok in the sail panel, door panel (like chido) or if they are very harsh, your speaker might have the option to mount them to the midwoofer cone itself (mb quart speakers have this, for example). then you can set the crossovers at -6db to turn the level down. mount metal tweeters away from places that can reflect sound
silk or soft-domes would be great (imo) in a custom molded a-pillar pod, since they are less harsh and would theoretically agree with your ears more
ill give you my honest opinion...ive got some infinity metal dome coax's in my car now (they were free and NIB, bring the flames
) and they HURT to listen to at loud volumes. i cant aim them down or forwards, and i have the HIGH setting at -1 as opposed to the stock speaker set at +3 on my pioneer. so even at their low position the soundstage is almost equal to stock, but the tweeter is really harsh. if your component set has a tweeter that sounds like this, youll want it aimed away from you lol.