What do you have already?
The best buys, IMO, are:
1) Ultimate DX set
Kind of expensive, but you get a ton of useful parts. Highlights are Duo, 230, WD, MB, LRF, and 90. Wing and L-Drago Guardian aren't half bad either.
2) Death Quetz
Again, lots of good parts here. Death and RDF are top-tier defense, and 125 is a good Attack part.
3) Flash Sagittario
Flash is top-tier Attack. Get this and the first item on this list, and you can make Flash Uranus (don't laugh) S130LRF.
4) The Strongest Blader set
Comes with Blitz Unicornio, Basalt Horogium, and Nightmare Rex. Blitz is top-tier Attack, and Basalt was top-tier Stamina and Defense. Blitz Unicornio is pretty good right off the bat, but needs RF, R2F, or LRF to be at its best.
4) Diablo Nemesis
Diablo is a great part. It is the best Anti-Attack part there is, meaning it has good Defense and a bit of Attack as well. However, since it's really screwy on the bottom, you really need to put BD145 on it. You can get that from Hell/Hades Kerbecs.
5) Don't buy a 4D beyblade.
Other valuable parts can be obtained from other beyblades, like Hades/Hell Kerbecs, (BD145 and Kerbecs) Dark Gasher/Cancer, (Gasher/Cancer and CH120) Ray Unicornio/Striker, (CS D125) and even Galaxy or Storm Pegasus, for R2F and RF, respectively.