Ludicolo is a nice alternative special sponge if you're sick to death of Blissey and Snorlax, and is a great wall to fellow Water-types, especially Swampert. Leech Seed is marvelous, and the surprising Rain Dance set hurts a lot if you're unprepared. However, it has a lot of trouble taking on Pokemon that pack Substitute or status moves.
Switch in on about any Water- or Ground-type, then mess around with whatever comes in. Toxic and Leech Seed pile on the residual damage quickly, and STAB Surf never hurts. Surf helps against most enemies, but Seismic Toss is its only hope against SubCM Raikou. Should you ever play 200, Protect will not only benefit Ludicolo by gaining Leech Seed recovery and Toxic damage, but will also save it from Slaking.
Use a Calm nature, as well as maximum HP and loads of Special Defense EVs, in order to reduce damage taken from special attackers it doesn't resist, such as Sceptile and Raikou. The rest should go in Special Attack.
This set can sometimes surprise your opponent if he or she expects the stall variant, and in particular destroys teams that rely on resistances instead of walls. Rain Dance makes Ludicolo faster than everything else, while Ice Beam lets Ludicolo take down Salamence. Leech Seed is for the obvious special wall, namely Snorlax or Blissey, while Hidden Power Grass can OHKO Swampert and beat up unsuspecting, low-health Water-types; moves from the Other Options section can be used here too. Lum Berry is a viable item if you don't want your sweeper to get paralyzed.
104 Speed EVs enable Ludicolo to beat Jolteon and Aerodactyl after the Swift Swim boost, as well as +1 Adamant Heracross. Maximize Special Attack because the point of a sweeper is to KO as many opponents as possible before being KOed. The rest should go in HP to optimize bulk.
ThunderPunch catches Gyarados off-guard, but will not OHKO it. Giga Drain is an option over Hidden Power Grass if you prefer the healing over PP and power. Fire Punch can hit both Steel- and Grass-types, which are immune to Toxic and Leech Seed, respectively.
Counter is not a bad idea; Ludicolo's average physical bulk should allow it to KO a heavy attacker. If using Counter, make sure to maximize HP and run about 200 Defense.
Checks and Counters
Venusaur, Tentacruel, Muk, and Gyarados are the best counters to Ludicolo. Venusaur, in addition to being immune to both Leech Seed and Toxic, is not even 2HKOed by sweeper Ludicolo's Ice Beam. Meanwhile, it can defeat Ludicolo with just two STAB Sludge Bombs. Tentacruel's Liquid Ooze ability makes Ludicolo's Leech Seed backfire; it packs STAB Sludge Bomb as well. Muk has high Special Defense and, once again, has STAB Sludge Bomb. Gyarados makes short work of the stall set with Taunt and STAB Hidden Power Flying, while the sweeper version lacks any effective move with which to take down Gyarados. However, if Ludicolo has ThunderPunch, Gyarados must predict well in order to come in safely.
To a lesser extent, the following options work too: Victreebel and Salamence (watch out for Ice Beam), Weezing (doesn't like STAB Surf), Blissey and Snorlax (hate Leech Seed), as well as anything with Toxic or Thunder Wave.