I could maybe see this claim being kind of valid with Flannery. Slugma does do a lot with Overheat but it needs Sunny Day to OHKO and Loudred will 2HKO if Slugma uses Sunny Day first. There is a possibility of missing the OHKO on Numel with Echoed Voice, though (62.5% chance to OHKO with 120 BP Echoed Voice assuming a Level 28 Loudred, but I guess one could justify that an extra level or two could secure it and is possible. By the way - the calculation did take Silk Scarf into account. Slugma might ruin it or help Loudred with Light Screen. Anyway, it's hard to really conclude whether Loudred is really victorious. Guess it's better than a lot of Pokemon have to say in this gym I guess, so we'll go with it.Then Altaria comes even later and now you have to grind for happiness to get a full power Return.
Anyway, defending my Whismur claim, to arms!
A lot of people are pointing out Loudred's poor early/mid-game (I'm ignoring Whismur since you are all dead right on that one) but you are forgetting one factor: Echoed Voice. While unimpressive at first, it will 2HKO anything unresisted (40bp followed by 80bp) on the first pokemon and from then on the pseudo-moxie effect kicks in and you start one-shotting the rest (120, 160, 200bp). This allows it to get boss kills that it honestly should not be getting (Flannery's Torkoal, Norman's Slaking's).
I'm not really sure on how you'll beat Slaking, though. Slaking's Retaliate alone does heavy damage to Loudred and you probably won't get very far in your Echoed Voices without healing Loudred on every loafing turn. Even if you did that Vigoroth or the other Slaking probably would come close to finishing you off (or will). Guess there's always the chance of Slaking wasting a turn with Yawn or Encore, but still Retaliate is probably what the AI will go for.
I would not mark Winona range whatsoever. This may be possible on a 3 man team (and if you had a 3 man team please say so); however, around Winona is pretty hard for a 4 man team. I would like to point out - I was about 7 levels below Level 40 when I reached Winona. I admit I had a team of 4.As for Exploud/Boomburst, you should be marking for Winona range to get it (Norman is way too early for the typical trainer, sorry RegiFlame150). Either right at or shortly after her. You'll have it for a while before Tate & Liza though.
Loudred's movepool is decent but it does come in at awkward times. This mainly applies to Surf and other moves like Flamethrower but Bite doesn't really hit much for SE damage (the rare Kadabra lurking around) at least until around Mt Pyre. Bulldoze, Rock Tomb, PuP, and I guess Overheat come at a good time (though Overheat can be a struggle in some of the routes due to the constant rain. Gamefreak y?). Otherwise you're looking at Stomp and Echoed Voice with the assistance of Secret Power (which probably would replace Stomp due to its superior secondary effects), the moves I mentioned (PuP, Bulldoze, Rock Tomb, Overheat after Gym 4) and that's about it.It's movepool is wide and never lacking at any point in the game. Stomp and Bite come early enough, Bulldoze at Mauville to crush Wattson, Overheat from Flannery (yes, Loudred can use Overheat), then Surf after Norman (and evolving). Add in Shadow Ball/Crunch later (unless you run into a scrappy one but that's DexNav so ignore this) and he's good to go. Has decent HM coverage, so he's a good support-mon in a game where that's more important.
Something I do want to point out is that as the game progresses Whismur does get exceptionally better, but I think people are really underestimating the amount of time to get to Exploud too (its EXP growth is not fast either). Like I said - if it's on a 3 man team please let me know so I can sort of re-evaluate.
I think my problem overall with Whismur is that it never really impresses until about Exploud territory. Though Echoed Voice was proven to help here and there (and I confess it can help in longer battles) the middling Speed and so-so durability also hampers its progress. Questionable evolution time also seems to be impending my side of the analysis (not questioning when we get Loudred - I'm questioning when we realistically get Exploud).The speed is a mitigating factor, enough to never support any S-tier claim, and Whismur is underwhelming before evolution.