A 98 The monster compendium lists all published monsters in 1st party and licensed 3rd party, including most stats. Below are the 1st party non-unique monsters with the highest stats. In italic are the monsters with the highest stats, excluding construct, elemental, outsider, undead.

Ability score <=10 HD <=15 HD <=20HD <=25HD <=50HD
Strength Ibrandlin (MoF, 10 HD, 35) Night Twist (MM3, 15 HD, 39) Greater Dusk Giant (HoH, 18 HD, 48) Nightcrawler (MM1, 25 HD, 48)
Ancient Night Twist (MM3, 25 HD, 45)		 Hagunemnon, Protean (ELH, 44 HD, 53)
Dexterity Glamer (Dr299, 10 HD, 30)
Will o' Wisp (MM1, 9 HD, 29)		 Kelvezu demon (MM2, 12 HD, 31)
Legendary Eagle (MM2, 12 HD, 30)		 Ha-Naga (ELH, 20 HD, 38) Gloom (ELH, 25 HD, 46) Tayellah (ELH, 34 HD, 48)
Constitution Caller from the Deeps (Storm, 10 HD, 30)
Whelp of Zargon (EEvils, 10 HD, 29)		 Vathugu demon (Dr337, 12 HD, 34)
Mountain Troll (MM3, 15 HD, 31)		 Titan (MM1, 20 HD, 39)
Hound of the Hunt (Wild Hunt) (MM5, 20 HD, 36)		 White Slaad (ELH, 24 HD, 37)
Ancient Night Twist (MM3, 25 HD, 34)		 Three-headed Sirrush (ELH, 45 HD, 49)
Intelligence Red Ethergaunt (FF, 5 HD, 23) Sarrukh (Serpent Kingdoms, 14 HD, 30) Ha-naga (ELH, 20 HD, 35) Mature Adult Obsidian dragon (Web, 25 HD, 27) Mature Adult Force Dragon (ELH, 50 HD, 42)
Wisdom Owl Archon (BoED, 10 HD, 25)
Threskisphinx (Sand, 10 HD, 24)		 Pleasure Devil, Brachina (FC2, 12 HD, 24)
Thoon Elder Brain (MM5, 12 HD, 23)		 Ha-naga (ELH, 20 HD, 35) Hellfire Wyrm (MM2, 23 HD, 26) Three-headed Sirrush (ELH, 45 HD, 43)
Charisma Falxugon (FC2, 9 HD, 22)
Gray Glutton (EPH, 9 HD, 20)		 Lilitu demon (FC1, 14 HD, 30)
Siabrie (Web, 15 HD, 25)		 Ha-naga (ELH, 20 HD, 36) Gloom (ELH, 25 HD, 46) LeShay (ELH, 50 HD, 47)

So yes, not only is the Titan the creature with the highest constitution and less than 20HD, it's also the highest-Con creature up to 25 HD, and even all the way up to 38 HD, only dethroned by the Uvuudaum. Also props to the Ha-Naga being an epic creature with 20 HD, meaning its absurd stats gave it the crown in four of the six ability scores.