Meganium is a good pokemon, and is definitely usable in the NU tier, it just has a lot of competition for bulky grass types. Its definitely better off since leafeon left, (it was basically an even defense leafeon without the attack or speed) but it still has a lot of competition. For the role of bulky grass type, it has to fight with Vileplume and Cradily, both of whom have certain advantages over the mono-grass type. Vileplume's poison typing eliminates a lot of its weaknesses, though it gives it trouble with some Slowking, most consider it a better typing. Vileplume gets Sleep powder over grasswhistle, (I hate 55%s...) but misses out on LS and reflect. Serebii says that it also misses out on leech seed, but I've seen sub-seeding Vileplume on shoddy so I know it can use it. They also share Aromatherapy, which is not as widespread as you might think. Lastly, their actual stats. Meganium is what you might call perfectly balanced. It's defensive stats are exactly balance, and its offenses are 2 points apart, given same EV's and IV's. It even has exactly the same speed as HP (I always considered those a pair 'cause of how everything else paired up. Vileplume, on the other hand, trades some of its bulk (especially physical) and a lot of speed for a 40-point special attack advantage. Really, these two come down to what you want in your grass type: a full on supporter, or a bulky tank that can dish out hits too?
Cradily, on the other hand, is another matter altogether. Grass is actually its secondary typing, so its more of a rock pokemon in the first place. That is probably why this thing lacks aromatherapy, but it makes up for it with bulkier stats than Meganium and a whole hoard of boosting moves. Also, its typing is fairly subpar, a mere two resists combined with 4 weaknesses, the main problem being the fighting one. This thing has a huge set of support moves, but almost none of them can be used to support the team. It gets recover, which is generally a better move than synthesis, especially in the sandstorm, Mirror Coat, ingrain, and every non-speciall boosting move on the planet. It's got Amnesia, Stockpile, Rock Polish, Swords Dance, Barrier, and Curse. the most useful ones are Stockpile (for a full out stalling set) SD (for immediate offense) and curse(used in the sandstorm, this thing is nearly unkillable. Not to mention it dishes out damage like Rhyperior, firing off SE and EQs left and right) Really, if your in the market for a supporter, Meganium's your girl, but if you want a boosting tank, go with the fossil.
On the offensive side, Meganium is outclasses more soundly. Victreebel uses SD as well, and Eggxecutor hits hard right off the bat, even though they both need Sunny Day to function at full capacity. Shiftry is basically a more offensive Meganium with SD, a much improved physical movepool, EXPLOSION, and even Leafstorm for an odd mixed set. Then there's Cacturne. Don't even get me started on Cacturne. Go get Luphrous, he loves Cacturne. Cacturne has an insanely high risk high yield set called sub-punch-punch. It involves sub, sucker punch, f(not for falcon)punch, and generally seed bomb, but you could make that SD if you were a raving madman. Basically, Cacturne is almost the pinnacle of offensive grass types, but he needs speed and has no way to get it, so he relies on substitute and Sand veil hax. With good prediction, this guy can sweep like never before, but you gotta be really, really good at predicting attacks if you're gonna use him.
Basically, if you want an offensive grass type use Shiftry. If you want a tank, use Vileplume. If you want a boosting tank, use Cradily. If you want an out and out supporter, use Meganium.
EDIT:Ninja'd by Ice-eyes. Not really surprising, considering how long it took me to write. Very good point, though. since the loss of Leafeon, we gained three superpowered fire-types, which basically but a lid on any grass-type attempts at taking Leafeon's place.
