What's more powerful. Self/ally speed buff or enemy speed debuff?

depend the moment.

as zilean , u can speed up adc or jungler to gank.

or slow opponent to stun him easily or to chase him easily.

Well both have soft caps on how useful they are, I was going to try to explain them but I think it would just be better to read.

http://leagueoflegends.wikia.com/wiki/Movespeed

Buff if their player is good debuff otherwise

Buffing ally movement speed, for a few reasons:

You can use the buff more often, because you can use it between fights, such as moving around the map or rotating between objectives. You're usually around allies more often than enemies (which is obvious if you can also cast it on yourself).

You can use movement speed buffs in order to allow someone to engage, disengage, help position easier, or to dodge skill shots. Slows are generally only used to help pick someone off or to help peel during a fight.

Unlike slowing an opponent, the buff duration is static assuming the character with the buff doesn't get stunned, knocked up, rooted, etc. Slows can be reduced by tenacity, cleansed, etc, and there are even a few items and runes that specifically combat slows (resolve runes, boots of swiftness, etc). That means slow durations are considerably weaker, and their actual effects are reduced.

Slows only slows walking speed. With how many champions and their grandmothers have a leap, dash, blink, teleport, etc, along with other tools to disengage like Trist ultimate, Vayne condemn, and so on forth, it's still easy for many champions still kite around even when slowed, and if they're looking for a fight, especially if you're melee and they are range, you will still be shot a few times before you can reach up to them. Movement speed buffs in contrast gives the opponent far less time to react, less time to attack once you enter their attack range, and any champion's positioning and disengage tools are not as potent when you can close the gap in far less time.

Movement speed buffs are not considered an attack as long as you don't hit an opponent first or are already in combat, so you do not enter combat. This is great for mobility boots (which gives you a lot of movement speed which generally works fantastically with any movement speed buffs that gives any kind of % based bonus), and that means you can do it in a bush, without being revealed and giving the enemy sight of that bush. This makes movement speed buffs fantastic for ganking.

It's variable, and it depends on circumstance, but generally:

Ally buffs are more utility focused whereas Enemy debuffs are better for almost every teamfight scenario.

As a Zilean main with literally thousands of games under my belt, I make this decision everytime I use my E.

It's almost always a better decision to slow an enemy if you have the choice.

I'd say depends on the champion and their kit. If they're a jungler a move speed buff would be especially better. If not, then I guess it depends. I do feel like slows have a much stronger effect than speed ups usually though. Even with my fastest move speed builds, one slow can really ruin you.