From the games thread, might find some of this useful.
"I am Super Vegeta" and "Super Saiyan Bargain Sale" both extend SSJ forms by a bit with a few stat boots.
However, Future Android 18's Z Soul "I"ll Use All My Strength to Kill You." automatically regenerates Ki, and actually holds the SSJ Transformation longer than either of the Super Saiyan Z Souls do. It's still a minor boost, but it is a bit longer. The only difference is you lose the bonus damage for Vegita's moves or General Ki attacks from the two SSJ related Z Soul's bonuses. Both of those bonuses are really pretty small anyway.
I think the extended life of the Transformation is more than worth the trade off to be honest. Future 18's Z Soul also grants an attack bonus when Ki is full, but generally speaking a Saiyan character isn't going to get much use out of that.
For non-Saiyan players not trying to keep a Ki draining transformation going, The Namekian Z souls tend to be the best. Starting with "Feel the Wrath of the Namekian, which is created from fusing "Don't Underestimate Earth" and "I must protect Elder Guru". I suggest skipping "A God Isn't What This World Needs" as it switches the health regen to when you use a support ability, and going straight to "I Am Neither Kami Nor Piccolo Now." which brings it back to when Ki is maxed. You do need "A God Isn't What This World Needs" to make that, but for anyone not needing to control Ki loss for a transformation it seems to have the most benefits for general use.
Also, if you have the DLC with GT Trunks, Pan, and Goku, then you get immediate access to two seven star level PQs that dish out about 40k xp with a Z ranking. The base reward is 25k zeni as well. I've only really messed around with the one where you fight Pan, Goku, and Trunks from GT. You need to beat Goku and Trunks before Pan to get the special conditions. It's a great way to level up if you've got a character and a couple of supporters that can deal with the level. Not for starters, but once you get to around lvl 30-40 you should be able to handle it if you're skilled enough. If not, around lvl 50 should be able to deal with it. You'll gain at least one level every time you beat it until around lvl 70.
The three characters are pretty strong, but not the strongest. GT Goku is neat because you can play around with SSJ 3, one of his versions has the ability. Unfortunately, he doesn't have many moves that can take much advantage of it, and his ki bar is very small. All three have nice move sets with the exception of Goku with SSJ 3. He has the Super Spirit Bomb, which takes a very long time, and covers a wide area, but you can only get about two off if you do them back to back with his SSJ 3 transformation.