I just finished Splinter Cell (the original...) for PC and I would NOT recommend a controller. It's certainly not required and the game is essentially a shooter. Analog input is not necessary as you actually control your speed manually (with the mouse wheel by default). Plus aiming will ALWAYS be easier with a mouse (a position offset will always be simpler than a velocity vector for a specified amount of time).
My roommate played PoP on the PC just a few months ago. It does not require a controller but your experience would definitely benefit from one.
EDIT: As for recommendations, I'm probably not a good source. I think the N64 controller is the pinnacle of flexibility. The X-Box controller S, however, is great when you need a second joystick instead of intuitive buttons (almost never with PC games). Because I prefer independence from unusual and non-standard input devices, I would not buy a "PC Game Pad." IF I ever buy a controller for my PC, it will be my favorite console controller and an adapter for it.