Shock Buffer

Description: This decreases the amount of damage taken by half, and keeps you from being knocked back after taking a hit.

On paper, this is a very useful upgrade, since tanking hits takes less time. However, this comes at the cost of dropping Hyper Dash, so you should never get this part.

Hyper Dash

Description: This makes your character dash faster.

This is, by far, the most useful part in the entire game. Routing MMX5 revolves around obtaining this upgrade as early as possible. This makes X or Zero dash much faster, as well as dash-jump much farther. Be wary that walljumping while holding the dash button makes them jump very far out, which can throw you off.

Super Recover

Description: This makes life and weapon refill capsules restore twice as much energy as normal.

Useful casually, but not very useful in speedruns. W-Energy Saver is much more useful, so get that instead.

W-Energy Saver

Description: This decreases the energy use of any Special Weapon or Technique.

Very useful, as it allows Zero to use Giga Attack and Dark Hold much more often. This part decreases WE usage by 25% if your character wears an armor or defensive upgrade, and 50% if they wear none.

Quick Charge

Description: (X only) This makes the X-Buster charge twice as fast.

Invaluable for X before he gets his ultimate armor. The other option from Squid Adler is near useless.

Z-Saber Extend

Description: (Zero only) This makes the Z-Saber project a small crescent wave in front of it to increase its range.

Nearly useless. The range increase is very, very small, and it does not help his aerial attacks either. Plus, the other option is the invaluable Quick Charge.

Burst Shots

Description: (X only) This allows you to have 5 buster shots on-screen at once instead of 3.

This gives X a much more rapid firing rate, which can be useful.

Shot Eraser

Description: (Zero only) This enables the Z-Saber to block most enemy shots.

This gives Zero more defensive power while navigating levels, which can be useful.

Anti-Virus Guard

Description: This increases your character's defense against the infection of the Sigma virus. The player will need to be hit by eight Sigma Virus drones before you get infected instead of four.

Useless. Don't bother. Though the other option is just as useless.

Virus Buster

Description: This enables your standard weapons to destroy Sigma Virus drones.

Useless. Don't bother. Though the other option is just as useless.

Jumper

Description: This makes your character jump higher and further.

Situational but not useless.

Speedster

Description: This makes your character run faster and jump further.

Somewhat useful. This improves X and Zero's speed when they're not dashing. Outclassed by hyper dash though, since you should almost always be dashing anyways.

Speed Shot

Description: This makes the X-Buster or Z-Buster's shots go faster; they'll fly across the screen faster.

They generally travel fast enough, so this is near useless. The other option is better.

Buster Plus

Description: This makes the X-Buster or Z-Buster do more damage.

Quite useful for X, especially in conjunction with other parts that improve his buster.

Ultimate Buster

Description: (X only) This makes every shot you fire fully charged, including special weapons (if you're using an armor that allows charging special weapons).

Very useful for X. However, keep an eye on your weapon energy.

Z-Saber Plus

Description: (Zero only) This makes the Z-Saber do more damage.

Invaluable for Zero, as this improves his SDC damage, allowing him to slay bosses noticeably more quickly.

References

1. ↑ Chart of MMX5 Parts at http://megaman.wikia.com/wiki/Parts#Mega_Man_X5_Parts_Charts and http://strategywiki.org/wiki/Mega_Man_X5/Parts