I notice you didn't mention if it was a Natural Cure or Poison Point Budew? That could make a difference.
I agree with Dragonman about Sunny Day + Solarbeam. I tend to regard it as a risky moveset that I'll try sometimes, but usually only put on a Fire-type. I really don't like Water Sport as a move - I don't think it helps much. The possible movesets I've got are far more pwnsome than lousy old Water Sport anyway.
Grass/Poison pokemon are weak to Fire, Ice, Flying, Psychic & Bug. I like to deal with as many vulnerabilities as possible. Roselia & Roserade naturally have much higher Sp.Atk than Attack, so I'd go for an all-Special moveset. Maybe something like:
1. Magical Leaf / Giga Drain / Energy Ball - GRASS, Special. Gets STAB.
2. Toxic - induces severe poison / Sludge Bomb - POISON, Special. Gets STAB.
3. Shadow Ball - GHOST, Special. Deals with the Psychic weakness.
4. Sludge Bomb - POISON // Weather Ball / Hidden Power - ???, Special
I have no preference between Magical Leaf, Giga Drain & Energy Ball. Magical Leaf & Giga Drain both have power of 90 with STAB - Magical Leaf is never-miss (useful with foes who spam evasion/accuracy loss) whereas Giga Drain gains you HP. Either is a good move. Energy Ball is obviously more powerful (120 with STAB), but requires a TM which you may not necessarily want to use.
I don't like Petal Dance because it locks you into the same move for 2-3 turns. So although it's more powerful, I wouldn't use it in competitive battling, because of the risk of someone switching in, say, a Grass Pokemon - which it'll do little damage to. You can have 2 rounds of the attack doing virtually no damage while you sit there seething because you can't change move. Nope, not a fan.
Toxic's special "extreme" Poison is nice, with the damage worsening each turn, but I don't know whether switching out reverts it to normal poison in the 4th Gen. The fact that Sludge Bomb gains the seriously nice power of 135 (with STAB) means you may prefer to use it, because it deals a lot more HP damage per round than even the "extreme" Poison status. Some people put both Toxic and Sludge Bomb in the same moveset - personally, I'm undecided. I'd definitely favour Sludge Bomb if I had to pick one or the other, though. And I seriously wouldn't bother using Toxic at all on a Poison Point Roselia.
Shadow Ball is a powerful Ghost-type attack, which is useful for dealing with Psychic attackers. It's not absolutely essential, but complements the other moves nicely if you don't already have a Ghost-type attack on your team. (I like to put Ghost-type moves on non-Ghost-type pkmn so they don't get massacred by their own same-type attack.) I don't think there's much you can do about Fire, Ice, Flying & Bug types except to use Sludge Bomb with its power 135, or going for one of the more "exciting" movesets below. Poison-type moves are only Super Effective against Grass Pokemon.
For the fourth move, it might be worth exploring Hidden Power. Before Diamond & Pearl I'd often see people suggesting movesets like "HP Grass". Now that it's become a bit easier to figure out your pkmn's IVs, it's possible you could work out what your Hidden Power type & strength would be before even putting it on the Pokemon. Failing that, there's the old "find a Kecleon & experiment" option. It's possible that your Budew's Hidden Power could be a decent Rock-type attack, which would deal with all of those remaining vulnerabilities! It could also be Fire, to hit Ice & Bug-types.
It's interesting that Roserade can learn Weather Ball. Previously, this was only available to Castform, where it was something of a "joke" signature move. The joke was on the opponent, though, because Castform's ability meant it would change type to a Type that always matched its Weather Ball, meaning you'd always get STAB. *considers migrating the Castform of pwnage over to Pearl*. Anyway... on Roserade, without Forecast ability, I'm not sure how useful Weather Ball would be - but you could potentially use it in combination with your own or a foe's Sunny Day/Rain Dance to take out Ice or Fire types. Here's how Weather Ball works:
Sunny Day: Weather Ball --> Fire-type, power 150. (Power doubles due to weather x 1.5 Sun bonus).
Rain Dance: Weather Ball --> Water-type, power 150. (Power doubles due to weather x 1.5 Rain bonus).
Hail: Weather Ball --> Ice-type, power 100. (Power doubles due to weather, no bonus).
Sandstorm: Weather Ball --> Rock-type, power 100. (Power doubles due to weather, no bonus).
As Sunny Day directly helps your opponents' Fire-types whereas Rain Dance doesn't help your opponents' Ice-types, I'd go for Rain Dance over Sunny Day. Thus, another decent moveset could be:
1. Magical Leaf / Giga Drain / Energy Ball - GRASS, Special. / Sludge Bomb - POISON, Special. Keep one STAB attack depending on the rest of your team.
2. Shadow Ball - GHOST, Special. Deals with the Psychic weakness. OR Choose the other Type STAB attack not used in 1.
3. Rain Dance. Changes Weather Ball type to Water.
4. Weather Ball - once rain is in play, WATER, Special. Power goes up to 150, Fire types = dead.
Hope this helps ;D.
