Raichu => BL
(And because I am discussing UU to BL, I'm able to talk about power and usefulness, right? Hopefully I understood Chaos correctly...)
Raichu is packing some serious S.Atk power behind already useful moves such as T.Bolt (STAB) and Surf, and he's got the speed to boot (And potentially sweep with a speed base of 100). Also, because it has a great move pool, Raichu can also make use of a Wish/Substitute strategy (As illustrated by the strategy analysis), as well as another strategy involving a fighting-type attack, Reversal. However, an Electric type armed with Surf (Especially a S.Atk stat like that) should be alarming enough, if not something to be highly aware of.
And unlike his pre-evolved form, Raichu's Atk stat is also very nice (mirroring his S.Atk at a suprisingly good base of 90), giving him lots of power behind a SubPunch strategy, not to mention that a well-timed Encore/Focus Punch combo could potentially turn the tides in a battle (Being that Raichu has already got some speed, oftentimes you can see which move you could Encore, as opposed to just predicting/blind guessing with slower pokemon choices). And on top of his good move pool, both his attack stats seem a cut above much of the UU crowd upon comparison. (His Atk stats actually suprised me when I first took a look at them. By no means should he even be assosiated with Pikachu...)
But since his HP and Def aren't too great, he requires some good prediction upon switiching in, making him tricky to use at times. But his great attacking stats, good speed, and diverse move pool seem to make Raichu very handy. Perhaps even a little too handy for hanging with those in the UU tier, seeing as how he can handle a multitude of types with the right moveset (Paired with his great offensive stats).
Rai may be underrated, but he is highly flexible when put in the right hands.
(The main beef with Rai is that he seems to have trouble with Grass and Dragon types... In that case, I suppose suppose you could always switch out another move for HP Ice if you so please...)
NOTE: Also, keep in mind that Reversal, Surf, and Wish cannot be used in the same moveset, being egg moves and whatnot. Therefore, it's important to pick one of Raichu's (potentially) best egg moves mentioned above, and build a strategy around it. However, Surf seems to be more of a stand-alone attack, and best fits with the SubPunch set metioned in the Strategy Analysis. Word.
EDIT: Base stats are the numbers within the colored bars in the Smogon Pokedex, right? Just want to make sure that I get my specs right.