Like Kyle said, Sagas is terrible. Don't touch it. I'd also avoid Super DBZ.
For the Budokai games, the original one goes up to the end of the Cell Saga with plenty of cutscenes in a fairly large amount of detail. It's a great game, but gameplay wise Budokai 2, Budokai 3, and Infinite World are better. The game is worth it solely because of the amount of detail in the cutscenes.
Budokai 2 can be skipped, but it's got a unique Dragon World mode, as well as a few things not seen in the later games for the characters that are available. Multiple different fusions like Tiencha and Gokule. Other than that, not really worth getting when Budokai 3 and Infinite World are better.
Budokai 3 and Infinite World are the two best. Reason I list both is because the gameplay system in Infinite World has been slightly enhanced, but also most characters combos have changed a little bit. They both have different capsule equipment menus. Infinite World has extra characters but a few are mission, etc. The story mode is different where both tell the entire storyline, Budokai 3 has where you go from place to place on the world map in 3d following the storyline out, yet it's fairly boring. Infinite World has the world map in 2d where you can go back to any mission you previously completed. It's got a lot of stupid mini games you have to go through other than fights.
Haven't played the PSP games so can't give an opinion on those, but I finally got Burst Limit for the PS3, which is basically similar to the Budokai games. After playing a bit of this game, I think it's overall worse than literally all of the Budokai games. It's like the changes that they made were exclusively to sap the fun out of the games. Good thing I got it for free with LA Noire so selling it works out nice.
For the Tenkaichi games. Make sure you start with the second. The first one sucks. Just like the regular Budokai 3, on Tenkaichi 2 you go around a 3d world map. However it's just locations. The script in battle is a couple lines and you just got the the fight. It goes in detail though that you fight most battles in the DBZ and GT Sagas. However, what's different with the Budokai games is that they play more like action games and if you want to feel like you're playing a Dragon Ball game, then you want to play the Tenkaichi games.
Budokai Tenkaichi 3 is the best of the Tenkaichi series though. While you don't go through most of the battles each saga like you do on Tenkaichi 2, the main thing on this one is you go through just the main battles and during battle the script follows closely to what it does on the anime. It's amazing and a must own if you're a DBZ fan.
This guy kind of sucks, but here's a Budokai Tenkaichi 3 video(Saiyan Saga against Vegeta.) It goes through the story specific parts during battle which make it much more fun than just fighting the enemy. http://www.youtube.com/watch?v=23kEP2NID2M
Raging Blast is a downgrade to Tenkaichi 3. It uses Tenkaichi 2's style where you go through battle after battle on each saga(and there are a ton of battles.) No stupid 3d map this time around thankfully. There are a few things tweaked but it's practically the same as Budokai 3 gameplay with a worse camera, far less characters(since it only goes up to DBZ,) and far less environments to battle on. It's like buying a sports game that has half the teams.
I didn't buy Raging Blast 2, but I only played the demo for a very short period of time and didn't like it. I'll eventually pick it up once it gets $20 or less, but I'd say avoid it if you're not sure about the series.