I don't think I actually own a copy of Digimon World anymore which is really unfortunate, I keep meaning to ebay one so I can play through again but I never actually get around to doing it. Before I got into any other Monster-themed game I had one of those Digimon tomagotchi clones that the raising system in Digimon World is based on which I think really boosts my nostalgia levels of that game a lot... it was the first RPG I ever beat, too. It amazes me how the devs decided to take such a radically different direction in future Digimon Worlds after creating such a great game...(I suppose MR2->MR3 had much the same problem).
I'd definitely check out Monster Rancher Advance/2 if you get the chance though, they're fairly similar to MR1 and 2 as far as gameplay goes, but the monster generation system is a lot more convenient(you enter letters/numbers into a tablet and the monster in generated based on what you input, as you gain breeder rank you can enter more characters for more powerful monsters) and the expedition system is a bit better, imo. They even bring some characters back from previous MRs(I forget which of the Advance games it is, but Holly returns from MR1(and the anime, lol) as an assistant) and generally seem to be aiming for the MR1/2 feel... if you get a chance to pick it up, do it.
Additionally while I am reminiscing about amazing monster themed games that should have sold much better than they did, Dragon Warrior Monsters is still my favorite game of all time I think. Pretty good breeding leveling system, the music was really solid for its time which is a lot of why I liked it I think, and the combat system was considerably better than MR, Digimon, or Pokemon's system IMO since it was pretty much just a slightly modified version of the traditional Dragon Quest/Warrior shit... I have a difficult time explaining it in a way that will do it justice but that's another game that probably costs
about three wads of gum and a paper clip now I would check out!
EDIT: And yeah Glalie, Zilla was the whale, he was my favorite species aesthetically I think although it was a little underwhelming in combat...
General Tso - Diva was in both of the Advance games, I know it was one of the last bosses, if not the last boss, in one of them but I forget the details exactly. Diva was the first and only monster I maxxed out on MRA2 thanks to exploiting a fairly nasty fatigue reducing bug.