There really isn't a 'best' ball formation; it's really character specific. Chipp, Millia, HOS, Zappa, I-No, and Faust beat far formations for essentially free (in my experience). Nevertheless, they can be useful in many situations.
I usually go, P ball, K ball, j.p (hits P ball) HS ball, land, D ball, S Carcass (makes balls fly around in a really good formation that can lock people down) HS ball, D ball, S ball, 5k S ball, K ball, j.k kick ball (or, if they try and jump in, j.H) air K ball, land, P ball, 5p.
At this point, the opponent is doing one of three things. If they try a jump-in, 6P, 6HS, CR. If they turtle, keep the balls flying. If they try and jump backwards, you can do my favorite. Chase the balls you it and summon a P ball at about half-screen so it hits the balls you already hit. Not only does it look pimp, but you can set up either a SA or a CR to try and guess where they go. I usually set a HS CR, charge it, and let it bounce. If they stay on the ground, 5p it at the last second to make it go forward and follow it in. Now they almost have to run away b/c you've got a beast of a pressure tool in front of you.
The first think I do is to make sure to summon a D and/or HS ball first, and then a P, K, or S ball to tag the both of them. Once you get either a K, P, 5p going, you can start using more balls or mix-up (lol) with P, K, jP, air dash in.
Oki games w/Venom are in all the combo vids...you could just check. I mean, the best I've seen involve K Dub, HS, tele, j.k, dash-jump k (hits HS ball and the other person, which you need to get the combo to work the best) whiff p in air, H (or S, depending on character height), continue.
Others include:
D Dub, P ball, 5k (hits P ball) tele, j.k...
HS Dub, dash-jump HS
(option one...land, 2D, SA, frc, combo)
(option two...air dash, S, HS, land, combo)
N-Otoko oki: P ball, 5p, S ball, 2s, S ball, 2s, S Carcass, HS Stinger
Ertai style (old school): P ball, 5p, S ball, 2s, S Carcass
Mizuchi style (mid school): HS Dub (hits) P ball, 5p (hits P ball), S ball, 2s, (hits S ball) SA (hits HS ball if you timed it right) dash, pressure
Mizuchi style revised: D Dub (hits) 5p, K ball, j.p (hits K ball), coming down, j.k (hits D ball) S ball, 2s, SA/CR
That's all I got for now.