Terran is better because:
- Surprise (cloak)
- Stim / concussive = slowing/haste
- High-damage units (siege tanks, banshees, thors)
- Repair/healing (scvs/medivacs)
- Unit synergies (marine/tank/medivac - gives mobility, drops, aoe, anti-air, healing, long range, and isn't weak to any one unit type).
- Mobile units (hellions, banshees)
- Ranged attacks > melee attacks
- No early-game weaknesses (MM are good against basically anything Tier1 and 2 - no glaring weaknesses (aside from Banelings))
- Very flexible openers (1 gas could mean anything, tech labs interchangeable.
- Uncounterable scouting (scan)
- MULES on gold patches.
- Come out ahead in base trades (flying buildings FTW)
- Easier to macro (can queue units instead of having to perfectly time your warp-ins or spawn larvae)
- Abilities to counter multiple special units (EMP)
- Early upgrades very powerful (stim, concussive)
- Ranged attack + small unit size = easy to bring units to bear (don't need concave as with bigger units)
- Ranged units are fast enough to kite, even without upgrades, with proper micro.
Terran's one weakness is the lack of mobility of their higher-tech units (Siege tanks, Thors, Battlecruisers), which explains why many Terran players stay with T1 units (marines, marauders) with upgrades, throughout the game.
Though other races have some very powerful units (Colossi, Carriers, Ultralisks, Broodlords), each of those have glaring weaknesses. Even their lower-tier units (roaches, stalkers, etc), have glaring weaknesses. Either they cannot hit air, or are crippled by their large size / surface area or are hard-countered by a "standard" T1 unit type the other side has. For example, MM beats Stalkers, Roaches. In Tier 2, Hydras die to Siege tanks, Roaches die to Immortals. Ultras and Broodlords get chewed up by mere Marauders and Marines, and Colossi die to anything air, and Carriers are nearly impossible to get out in quantity and are slow.
So... Terran are the most flexible, powerful race... if you can use them properly. ;-)