Mainly the maps, weapons, and gameplay speed. It's not even a case of liking it most due to it being the first I played, because I played 2 first.
Obviously the sequels do things better, especially in regards to the single player side of things, thanks to arcade leagues and an actual story mode, but I just prefer the core gameplay of the original, and always have done.
Planet X remains one of the best multiplayer maps ever created in my opinion, and it's a shame it never reappeared in the sequels.
Quite a fair reasoning! I cannot say much about TS1, I only played it for the first time in recent years, getting 100% was no problem but the MP didn't keep me too hooked, though from having the trilogy extensively, beside 1, each of 'em got their strength and weaknesses.
Timesplitters: I already said I didn't play it too much but enough to have a good idea.
+ It has some excellent multiplayer maps, Castle, Graveyard, Planet X (biggest map of the entire trilogy)
+ Capture The Bag with 4 teams (instead of Red vs Blue in 2 and onward)
+ A very basic mapmaker but with 2 standout tileset, gothic and alien, I especially love the big spaceship that appeared in the very big room.
+ Tons of unlocks
- The multiplayer itself was kinda so-so, one thing I didn't like was bots starting unarmed, it's a very minor nitpick and sure you can handicap yourself too but...
- insane damage, varying between dying instantly or only in a few shots
Timesplitters 2:
+ A slightly better story mode but still very minimal
+ Some solid multiplayer maps again
+ Easily the hardest AI for multiplayers, which you can tweak via 3 difficulties settings and stars for each bots
+ Even more unlocks, but this time maps included as well and cheat codes for silliness purpose (also bricks!)
+ Arcade Leagues/Challenges
- Mapmaker is improved but none of the tileset really stood out. You can create story missions but it's very basic and you're limited to only 10 enemies.
- Story mode can be helluva challenging on hard, every single bullets you take can mean the difference between surviving or dying because there's zero health packs, only armor (though 1 mission offer a full life refill)
Timesplitters Future Perfect:
+ Robust story mode with lots of dialogues, actual characters, humors and all that jazz. Definitely a high point.
+ A plethora of characters, 149 in fact!
+ The best mapmaker to date, although again none of the tilesets really stick out but you can remove ceiling to have skyboxes, you can have rain, snow, etc. You can also overlap tiles so now rooms can be 6 stories tall. Story creation severely improved with up to like 50 enemies, if not more.
- The multiplayer maps this time around aren't nearly as good as they were in TS2 and 1, there's a few standouts but most of the new ones are...forgettable.
- Game felt a bit rushed, I played it on Gamecube but I recall experiencing a lot of freezing. Mapmaker was limited to only 6 bots instead of the usual 10 on GC and PS2, didn't apply to Xbox where everything was intact. Lots of issues in MP too such as...bots going through windows (if your created maps had them), in Siberia if you went super far from the fight, bots had the weird habit of bunching up together...and just die as soon as you got close to the fight
- AI was much, much easier in multiplayer. Because they're better animated a lot of time you shoot at them and just stagger on place for some easy kills, which never happened in TS2 where bots were a lot more oppressive on frantic 3+ stars.
I did not include the shooting because I often wonder which I like more...the more arcadey feel of TS1/TS2 or TSFP. The first two games does feel a lot faster on that end so...hmmm.